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# Maths...

Registered User, __BANNED USERS regular
edited November 2010
Fffff...
I am going to take a college placement test tomorrow (well sometime this week) and I am going over basic math before I move onto the other stuff. Anyways I have to find what 11 percent of 3000 is, I can do it in my head but that probably won't help me on the test if they have some ridiculous number, I don't want to waste too much time.

Anyways I had to look up how to do long division on paper, I have been using a calculator and doing the rest in my head since about sixth grade when we stopped using that crap. So how the hell do I do this? The practice guide has some bullshit equation where they do this...

Example: 20% of what number is 16?

.2*a=16

(20/100)a=16

1/5a=16/1

5* (1/5)a = (16/1) * 5

a=16*5
a=80

What the bearcock just happened? They explain nothing except that I need to multiply both sides of the equation by five, what a load of shit. Why multiply? Why 5? Why both sides? Why the fuck don't math books ever explain the reasoning behind anything.

Anyways all I need to know is what I do with 11? Five obviously doesn't work so what do I do?

11/100 doesn't reduce all cool like the other one so do I just do times 100 on both sides? This doesn't seem right. Multiply both sides by 100, now I have that same thing had before except with more zeros. So I still have to do 11/3000 so I really got nowhere as far as I can tell.

Fizban140 on

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unionize your workplace Seattle, WARegistered User regular
edited November 2010
Lemme see if I can help.

You multiply both sides because it keeps the equation equal. x=y is equivalent to 5x=5y.

The reason you multiply, and the reason you multiply by 5 specifically, is because in order to get the answer you need to go from:

.2*a = 16

to

a = ???

Which you do by converting .2a to 1, which you do by multiplying by 5.

The "trick" to do this is convert it to a fraction (1/5) then turn that fraction upside down (5/1) and multiply by that number.

If all that makes sense, you should have an idea how to do the 11% of 3000 problem. If it doesn't, it's because I haven't had to teach math in aaaaages, so don't blame yourself.

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Registered User, __BANNED USERS regular
edited November 2010
I get that part sort of I just don't understand how I can apply it to 11 percent of 3000.

11/100 just ends up turning into 11=3000

Fizban140 on
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Registered User, __BANNED USERS regular
edited November 2010
I don't understand how to do .11 * 3000 I guess then.

Fizban140 on
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San DiegoRegistered User regular
edited November 2010
You have to set up the equation according to the problem being asked:

What percent of 3000 is 11?
3000x = 11, where x is going to be a percent value

What is 11% of 3000?
(0.11)(3000) = x

3000 is 11% of what number?
0.11x = 3000

Orogogus on
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unionize your workplace Seattle, WARegistered User regular
edited November 2010
Hmm. It actually doesn't apply at all. 11% of 3000 is just a fractions problem.

11/100 * 3000/1 = 33000/100 = ?

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Registered User, __BANNED USERS regular
edited November 2010
Fuck I got it I guess, I need to learn long multiplaction too.

That example problem didn't help at all with the practice problem directly following it.

Fizban140 on
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Registered User, ClubPA regular
edited November 2010
Fizban140 wrote: »
I don't understand how to do .11 * 3000 I guess then.

3000
x .11
-----
30.00
+300.00
-------
330.00


Doc on
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unionize your workplace Seattle, WARegistered User regular
edited November 2010
Orogogus wrote: »
You have to set up the equation according to the problem being asked:

What percent of 3000 is 11?
3000x = 11, where x is going to be a percent value

What is 11% of 3000?
(0.11)(3000) = x

3000 is 11% of what number?
0.11x = 3000

Erm. That's not even remotely the same. :P

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unionize your workplace Seattle, WARegistered User regular
edited November 2010
Fizban140 wrote: »
Fuck I got it I guess, I need to learn long multiplaction too.

That example problem didn't help at all with the practice problem directly following it.

That sounds like a math textbook.

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Registered User regular
edited November 2010
The two examples you gave are asking slightly different things. The first question is: what is 11% of 3000? The second question is: 16 is 20% of what number? Be sure to read the questions carefully so you know what they are asking.

For the first question:

11% is the same as 11/100. So, to find 11% of 3000, multiply 3000 by 11/100.

3000*(11/100) = 330.

For the second question:

20% is the same as 20/100. 20% of the variable a is (20/100)*a. So, to find the number that gives 16 when you take 20% of it, set up this equation:

(20/100)*a = 16.

To solve this equation for a, you need to rewrite the equation so that a is alone by itself on one side of the equation. To do that, you need to get rid of the (20/100) which is currently on the left side of the equation. To get rid of the (20/100), multiply both sides of the equation by the reciprocal (the fraction flipped upside-down).

(100/20)*(20/100)*a = (100/20)*16.

(100/20)*(20/100) is just 1. So the equation above simplifies to

1*a = (100/20)*16.
a = (100/20)*16.
a = 80.

In your textbook, they multiplied both sides of the equation by 5. That works too, because 5 is equal to 100/20. 5 was chosen because it is equal to 100/20, the reciprocal of 20/100.

Heres another equample: 88 is 11% of what number?

11% is the same as 11/100.

(11/100)*a = 88.

Get rid of the (11/100) on the left side of the equation by multiplying both sides of the equation by the reciprocal, (100/11).

(100/11)*(11/100)*a = (100/11)*88.
1*a = (100/11)*88.
a = (100/11)*88.
a = 800.

Hirocon on
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San DiegoRegistered User regular
edited November 2010
Orogogus wrote: »
You have to set up the equation according to the problem being asked:

What percent of 3000 is 11?
3000x = 11, where x is going to be a percent value

What is 11% of 3000?
(0.11)(3000) = x

3000 is 11% of what number?
0.11x = 3000

Erm. That's not even remotely the same. :P

That's, uh, the point. I think the reason he can't get from the example problem to the one being asked is because this isn't clear. They're different problems, and you have to look at the phrasing to see why.

Orogogus on
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Registered User, __BANNED USERS regular
edited November 2010
Ok I just got that, I am going to quite for the night I am too tired if I am making that stupid of mistakes.

Why is it that every math book is set up perfectly for you to hate it just a little bit more every single time you use it? It really is incredible.

Fizban140 on
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unionize your workplace Seattle, WARegistered User regular
edited November 2010
Orogogus wrote: »
Orogogus wrote: »
You have to set up the equation according to the problem being asked:

What percent of 3000 is 11?
3000x = 11, where x is going to be a percent value

What is 11% of 3000?
(0.11)(3000) = x

3000 is 11% of what number?
0.11x = 3000

Erm. That's not even remotely the same. :P

That's, uh, the point. I think the reason he can't get from the example problem to the one being asked is because this isn't clear. They're different problems, and you have to look at the phrasing to see why.

I gotcha.

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Registered User regular
edited November 2010
The easiest way to do these types of problems is to set them up this way:

11% of 3000?

11/100 = x/3000

notice how I put the equation in terms of

part/whole = part/whole

20% of what number is 16?

20/100 = 16/x

solve for x.

Try and relate them that way, it might make more sense.

Demerdar on
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Registered User new member
edited November 2010
dude its 330 .. don't be more confused just use the reciprocal procedure and you will get accurate value.
I mean take all steps from end to start .. got it???

shawnchin on
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Registered User, __BANNED USERS regular
edited November 2010
I have no idea what the reciprocal procedure is.

Fizban140 on
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!!! Registered User regular
edited November 2010
I use cross multiplying for figuring out percentages. Here's how it works:

First, we know that non-reduced fractions are the same as their reduced versions, right?
So

1/2 = 2/4 = 4/8 = 100/200 and so forth.

To cross multiply, simply take the numerator (top number) of one fraction and multiply it by the denominator (bottom number) of the other fraction. In equal fractions, the multiplied answers will be the same.

For example:
1    2
- = --
2    4


Cross multiplying you get:
1x4 = 4
2x2 = 4

To figure out a percentage of a number, you can use cross multiplication to easily get your answer.

Let's try with something simple first.

"What is 50% of 10?"
 50      x
--- = --
100    10


100 * x = 50 * 10
100x = 500
x = 5

Let's try it the other way.

"10 is 50% of what number?"
 50     10
--- = --
100     x


50 * x = 10 *100
50x = 1000
x = 20

Now let's try something more difficult.

"What is 34% of 207400?"
 34     x
--- = --
100   207400


100 * x = 34 * 207400
100x = 7051600
x = 70516

Hope this helps.

SeñorAmor on
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drinking coffee in the mountain cabinRegistered User regular
edited November 2010
11% of 3000 is what number?

11/100 * 3000 = x.

20% of what number is 16.

20/100 * x = 16.

"What number" means x. "Percent of" means "/100 *".

You then divide both sides by the same number, or multiply both sides by the same number, until you get X by itself.

The first problem would go like this:

20% of what number is 16
Use the direction I gave above to translate this into an equation

20/100 * x = 16.
We want to move things to the right and leave x on its own side of the equation.
Multiply both sides by 100 to get rid of the "/100" on the left side.

20 * x = 16 * 100
Halfway there, we need to move the "20*" on the left over to the right side.
Divide both sides by 20.

x = 16 * 100 / 20.
Now it's straightforward multiplication and divison. In this case there is a shortcut of noticing that 100/20 = 5, so you can just do 16*5.

Powerpuppies on
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Registered User regular
edited November 2010
easy way to remember how to set up any % problem
%          is part
---    =  --------
100       of whole


Since these questions pretty much have to use these words it's pretty simple to do the substitution. After that the cross multiplication seems to be covered (I hope I haven't read all of these posts thoroughly)

musanman on
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Registered User regular
edited November 2010
A percent is just another way of saying a fraction.

66% of something is roughly two-thirds, 2/3
50% of something is a half, 1/2
25% of something is a quarter, 1/4
20% of something is a fifth, 1/5
10% of something is a tenth, 1/10

If a problem asks you what is X% of SomeNumber, you should be able to immediately ballpark an answer.

27% of 8300? I dunno exactly, but 27% is pretty close to a fourth, and a fourth of 8000 is 2000, so the answer should be around 200.

Write it down, use it to sanity check the real answer you get. (And if you're really in a time pinch, use it to guess the right answer :P)

To get the real answer, just shift some decimals and multiply.

27% of 8321 in equation form is .27*8300
Whenever I see equations where I have decimals, I like to shift it so the equation lacks decimals. 27*83 is the same as .27*8300, but it's much more palatable to mental math.

Doing 27*83 gets 2241. That's in line with our eyeball answer, 2000, so all is good.

TechBoy on
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Registered User, __BANNED USERS regular
edited November 2010
SeñorAmor wrote: »
I use cross multiplying for figuring out percentages. Here's how it works:

First, we know that non-reduced fractions are the same as their reduced versions, right?
So

1/2 = 2/4 = 4/8 = 100/200 and so forth.

To cross multiply, simply take the numerator (top number) of one fraction and multiply it by the denominator (bottom number) of the other fraction. In equal fractions, the multiplied answers will be the same.

For example:
1    2
- = --
2    4


Cross multiplying you get:
1x4 = 4
2x2 = 4

To figure out a percentage of a number, you can use cross multiplication to easily get your answer.

Let's try with something simple first.

"What is 50% of 10?"
 50      x
--- = --
100    10


100 * x = 50 * 10
100x = 500
x = 5

Let's try it the other way.

"10 is 50% of what number?"
 50     10
--- = --
100     x


50 * x = 10 *100
50x = 1000
x = 20

Now let's try something more difficult.

"What is 34% of 207400?"
 34     x
--- = --
100   207400


100 * x = 34 * 207400
100x = 7051600
x = 70516

Hope this helps.

That helps quite a bit, but I am sure I will fuck up on where to put the x since I don't quite understand how all this works.

Math really isn't my thing.

So 6% of 50 would be

6/100 = 50/x?

20 is what percentage of 50?

50/100 = 20/x?

Fizban140 on
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Registered User regular
edited November 2010
Fizban140 wrote: »
SeñorAmor wrote: »
I use cross multiplying for figuring out percentages. Here's how it works:

First, we know that non-reduced fractions are the same as their reduced versions, right?
So

1/2 = 2/4 = 4/8 = 100/200 and so forth.

To cross multiply, simply take the numerator (top number) of one fraction and multiply it by the denominator (bottom number) of the other fraction. In equal fractions, the multiplied answers will be the same.

For example:
1    2
- = --
2    4


Cross multiplying you get:
1x4 = 4
2x2 = 4

To figure out a percentage of a number, you can use cross multiplication to easily get your answer.

Let's try with something simple first.

"What is 50% of 10?"
 50      x
--- = --
100    10


100 * x = 50 * 10
100x = 500
x = 5

Let's try it the other way.

"10 is 50% of what number?"
 50     10
--- = --
100     x


50 * x = 10 *100
50x = 1000
x = 20

Now let's try something more difficult.

"What is 34% of 207400?"
 34     x
--- = --
100   207400


100 * x = 34 * 207400
100x = 7051600
x = 70516

Hope this helps.

That helps quite a bit, but I am sure I will fuck up on where to put the x since I don't quite understand how all this works.

Math really isn't my thing.

So 6% of 50 would be

6/100 = 50/x?

20 is what percentage of 50?

50/100 = 20/x?

No, not quite.

6% of 50 would be

6/100 = x/50

say it out loud:

"six out of one-hundred is equal to 'what' out of 50?"

20 is what percentage of 50?

x/100 = 20/50

Kind of like... 50 is what percentage out of 50?

x/100 = 50/50

where x = 100

100/100 = 50/50

1 = 1

So, 50/50 = 100%

Hopefully this helped a bit. Kind of hard to teach this kind of thing over the internet.

Demerdar on
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... TorontoRegistered User regular
edited November 2010
I like breaking those operations down into multiples of 10% (the remainder being multiples of 1%).

1% of 50 is 0.5, so 6% is six times that. 6 times 0.5 is 3.

EDIT: I should mention that this works best for round numbers, like 3000 or 50. Although it will be harder to do something like x% of 43, its not impossible to do so with this method. As with every method in math, your mileage may vary.

finnith on
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Registered User, __BANNED USERS regular
edited November 2010
Hope no one saw that, I got it, that was a really fucking stupid mistake I made.

Fizban140 on
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Registered User, __BANNED USERS regular
edited November 2010
Demerdar wrote: »
Fizban140 wrote: »
SeñorAmor wrote: »
I use cross multiplying for figuring out percentages. Here's how it works:

First, we know that non-reduced fractions are the same as their reduced versions, right?
So

1/2 = 2/4 = 4/8 = 100/200 and so forth.

To cross multiply, simply take the numerator (top number) of one fraction and multiply it by the denominator (bottom number) of the other fraction. In equal fractions, the multiplied answers will be the same.

For example:
1    2
- = --
2    4


Cross multiplying you get:
1x4 = 4
2x2 = 4

To figure out a percentage of a number, you can use cross multiplication to easily get your answer.

Let's try with something simple first.

"What is 50% of 10?"
 50      x
--- = --
100    10


100 * x = 50 * 10
100x = 500
x = 5

Let's try it the other way.

"10 is 50% of what number?"
 50     10
--- = --
100     x


50 * x = 10 *100
50x = 1000
x = 20

Now let's try something more difficult.

"What is 34% of 207400?"
 34     x
--- = --
100   207400


100 * x = 34 * 207400
100x = 7051600
x = 70516

Hope this helps.

That helps quite a bit, but I am sure I will fuck up on where to put the x since I don't quite understand how all this works.

Math really isn't my thing.

So 6% of 50 would be

6/100 = 50/x?

20 is what percentage of 50?

50/100 = 20/x?

No, not quite.

6% of 50 would be

6/100 = x/50

say it out loud:

"six out of one-hundred is equal to 'what' out of 50?"

20 is what percentage of 50?

x/100 = 20/50

Kind of like... 50 is what percentage out of 50?

x/100 = 50/50

where x = 100

100/100 = 50/50

1 = 1

So, 50/50 = 100%

Hopefully this helped a bit. Kind of hard to teach this kind of thing over the internet.
I absolutely do not understand this at all, it doesn't even feel like english.

Fuck I think I need to take a break already, this is so humiliating.

Fizban140 on
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... TorontoRegistered User regular
edited November 2010
So first you have to find the common denominator. I usually go through the multiples of the highest denominator (in this case it 8) until I find one that is divisible by the other denominators. 24 is the lowest common denominator in this case.

(1/3)*8=8/24
(3/8)*3=9/24
(1/4)*6=6/24

Note that I just multiplied the fractions by the amount it takes to get the denominator to equal 24.

(8+9+6)/24=23/24

So it's d

finnith on
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Registered User regular
edited November 2010
Fizban140 wrote: »
Demerdar wrote: »
Fizban140 wrote: »
SeñorAmor wrote: »
I use cross multiplying for figuring out percentages. Here's how it works:

First, we know that non-reduced fractions are the same as their reduced versions, right?
So

1/2 = 2/4 = 4/8 = 100/200 and so forth.

To cross multiply, simply take the numerator (top number) of one fraction and multiply it by the denominator (bottom number) of the other fraction. In equal fractions, the multiplied answers will be the same.

For example:
1    2
- = --
2    4


Cross multiplying you get:
1x4 = 4
2x2 = 4

To figure out a percentage of a number, you can use cross multiplication to easily get your answer.

Let's try with something simple first.

"What is 50% of 10?"
 50      x
--- = --
100    10


100 * x = 50 * 10
100x = 500
x = 5

Let's try it the other way.

"10 is 50% of what number?"
 50     10
--- = --
100     x


50 * x = 10 *100
50x = 1000
x = 20

Now let's try something more difficult.

"What is 34% of 207400?"
 34     x
--- = --
100   207400


100 * x = 34 * 207400
100x = 7051600
x = 70516

Hope this helps.

That helps quite a bit, but I am sure I will fuck up on where to put the x since I don't quite understand how all this works.

Math really isn't my thing.

So 6% of 50 would be

6/100 = 50/x?

20 is what percentage of 50?

50/100 = 20/x?

No, not quite.

6% of 50 would be

6/100 = x/50

say it out loud:

"six out of one-hundred is equal to 'what' out of 50?"

20 is what percentage of 50?

x/100 = 20/50

Kind of like... 50 is what percentage out of 50?

x/100 = 50/50

where x = 100

100/100 = 50/50

1 = 1

So, 50/50 = 100%

Hopefully this helped a bit. Kind of hard to teach this kind of thing over the internet.
I absolutely do not understand this at all, it doesn't even feel like english.

Fuck I think I need to take a break already, this is so humiliating.

Give it some time to digest. Think about what we are saying and intermittently throughout the day go through it in your head. It will click and you will /facepalm yourself.

Remember it's all about parts of a whole. "PARTS OF A WHOLE".

Demerdar on
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Registered User regular
edited November 2010
Fuck I think I need to take a break already, this is so humiliating.

I worked in a Math Lab for years; you're doing about as well as anyone not "dialed in. . ." to math does. It isn't even close to being "humiliating".

I would suggest checking out Khanacademy.org. He most definitely has a video on percentages (and other algebra subjects). I would say the most important thing you should take from all of this is how to convert a word problem into an equation, and that comes with recognizing what words "mean". "Is" is "=" and "of" is "*" in this case (and generally all cases). The related ratios approach is also useful, but that only works if you both understand and agree that it works. You may not be there yet.

ED! on
"Get the hell out of me" - [ex]girlfriend
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Registered User, __BANNED USERS regular
edited November 2010
Well I took the test and I failed the algebra part, which sucks because I thought I did well on it. I think I just went too fast and messed up, oh and I forgot that foil was a thing, thought it might have been fil so I failed those questions pretty hard.

Anyways I have been taking practice tests and I can't get 25 or 30.

http://webs.anokaramsey.edu/bordwell/Accuplacer%20Study%20Material/elementary_algebra_test.pdf

Fizban140 on
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Registered User regular
edited November 2010
Fizban140 wrote: »
Well I took the test and I failed the algebra part, which sucks because I thought I did well on it. I think I just went too fast and messed up, oh and I forgot that foil was a thing, thought it might have been fil so I failed those questions pretty hard.

Anyways I have been taking practice tests and I can't get 25 or 30.

http://webs.anokaramsey.edu/bordwell/Accuplacer%20Study%20Material/elementary_algebra_test.pdf

I will use multilevel spoilering here, with a warning before the final (solution) spoiler.

25:
Break the problem down in to parts. You want to know how far the bike goes in a given period of time, and you know how many revolutions it goes in a (different) given period of time. So, calculate how far it goes in one revolution, then calculate how many revolutions it goes in one minute, then multiply the two. Make sure to convert the answer to meters!
To find out how far the bike goes in one revolution, calculate the diameter of the bike wheel.
To find out how many revolutions it goes in one minute, take the number of revolutions per second times 60 (seconds per minute)
Don't forget to convert it to meters! Divide the distance you have by the number of centimeters in a meter.
FINAL SPOILER
The diameter of the bike wheel is 2*pi*r; we know the diameter is 70cm, so the radius is 35cm, so 2*pi*r = 2*pi*35 = 70*pi centimeters. The number of revolutions per minute is 3 (revolutions per second) * 60 (seconds per minute). Thus the answer is 70*pi*3*60 centimeters, or 70*pi*3*60/100 meters.

30:
First, the algebraic way:
You want to use some facts about geometry to come up with two equations, then solve for x and y. At that point, you can plug your values for x and y in to the equations given by the problem and see which one is not true.
y + (3x + 10) = 180, because that is a straight line.
x + y = 90, because the angles in a triangle add up to 180 and one of the angles in this triangle is a right (90 degree) angle.
Rearrange the first equation to get 3x + y = 170.
FINAL SPOILER
Subtract the equation x + y = 90 from the equation 3x + y = 170 to get 2x = 80. Then x = 40. Substitute this in to x + y = 90 to get 40 + y = 90, or y = 50. Then substitute these values in to the equations to see that x = (3x + 10) is the one that fails.

Second, the non-algebraic but probably unhelpful way:
Notice that if x = 3x + 10 is true (equation D), then x and 3x + 10 are either both acute, both right, or both obtuse.
x and y must be acute for the figure to be a triangle, because x > 0, y > 0, and x + y + 90 = 180, so x + y = 90. Thus, 0 < x < 90 and 0 < y < 90.
y and 3x + 10 add up to 180, because the horizontal line is... a line.
If y < 90, and y + 3x + 10 = 180, then 3x + 10 > 90, meaning 3x + 10 is obtuse.
FINAL SPOILER
We already know (from the outermost spoiler) that x is acute, and now we know 3x + 10 is obtuse. But then x = 3x + 10 can't possibly be true! So, equation D is incorrect.

Clipse on
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Pirate Ninja Portland, ORRegistered User regular
edited November 2010
When I see problems like "What is 11% of 3000?" on an entrance/placement exam, it sends red flags to my head. The problem tells me the test makers are separating the people who CAN do the math to people who know HOW to do math.

You can go do the traditional long route (more time consuming), or be a little clever (and save time). Easiest way would be breaking down the percentage.

3000 * (10/100 + 1/100) = 3000 * (10/100) + 3000 * (1/100) = 330

In other words 10% of 3000 is 300. 11% of 3000 is 300 plus a tenth of 300 (30).

For example, what is 17% of 3000?

3000 * (10/100 + 7/100) = 3000 (10/100 + 7 * 1/100) = 300 + 7 * 30 = 300 + 210 = 510

At least for me, breaking it down like that makes it a lot easier to do basic mental math. Definitely helped me during the years I was in college when I was studying Chemical Engineering

VeeSee on
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Pirate Ninja Portland, ORRegistered User regular
edited November 2010
Eh.. Just saw finnith described the same method. High-five sir.

VeeSee on
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Registered User regular
edited November 2010
For #30, another useful relationship to remember is this: The angle of measure 3x+10 is called an exterior angle. Basically, that's the angle you get when you extend one side of a triangle out.

The measure of an exterior angle of a triangle is equal to the sum of the two remote interior angles; that is, the two interior angles which don't touch the exterior angle. This means that 3x+10=X+90 in that diagram. So it can't be true that x=3x+10.

Just be careful, this ONLY applies to triangles. If they throw an exterior angle of a quadrilateral at you, it won't work.

GoodOmens on

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