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percent increase (Q 26 on CAT 3)

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deborah
 Post subject: percent increase (Q 26 on CAT 3)
 Post Posted: Sun Nov 23, 2008 11:14 pm 
x% of y is increase by x%. What is the result in terms of x and y?

a) 100xy + x
b) xy + x/100
c) 100xy + x/100
d) 100xy + xy/100
e) xy(x+100)/10000

The solution substitutes x=10 and y=100 but I wouldn't have selected those numbers because the strat guide says to choose prime numbers when substituting so I tried x=5 and y=7. Can you show me what the basic equation is to solve this equation? I tried to solve it by using this equation:

xy/100 + x/100

thanks.
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Shivreet Sekhon
 Post subject: Reply
 Post Posted: Mon Nov 24, 2008 2:00 am 
The answer would be

((x/100) * y) + (x/100) ((x/100) *y)

First term is x% of Y

Second term is increase of x% of the first term

Is the answer E
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esledge
 Post subject:
 Post Posted: Sat Dec 13, 2008 7:12 pm 
Offline
ManhattanGMAT Staff


Posts: 901
Location: St. Louis, MO
You don't have to use prime numbers when picking numbers. That's just a suggestion to help you avoid picking numbers that are multiples of each other--sometimes multiples can lead to two choices giving the target number.

On percent problems, just pick numbers that are easy percents to take (usually multiples of 10), and easy to take percents out of (usually 100). I would avoid 50% as a picked number, just because it is exactly half, so you can lose track of the percent you took vs. the percent that is left over.

Shivreet's algebra is correct, and (E) is the right answer.

Notice that the answers generally differ on the placement of a factor of 100. This is a very easy trap to fall into with algebra. Be careful if you take that approach.

_________________
Emily Sledge
Instructor
ManhattanGMAT
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