Hi,

This is a question from my CAT2:

At the beginning of 2010, 60% of the population of Town X lived in the south and the rest lived in the north. During 2010, the population of Town X grew by 5.5%. If the population in the south grew by 4.5%, by how much did the population in the north grow?

(A) 1%
(B) 3.5%
(C) 6.5%
(D) 7%
(E) 13.75%

I had the same approach with the alternative way of the explanation (choose total to be 100, or 1000), but I didn't choose the total number of people, I chose the total percentage of the population to be 100%.

Before: 100%
South = 60%
North = 40%

After: 105.5%
South = 60% + 60% x 4.5% = 62.7%
Therefore North = 105.5 - 62.7 = 42.8%
So North increase = 42.8 - 40 = 2.8%.

Now I don't see any choice to be 2.8%. I re-read the question and it states "by how much did the population in the north grow?". Therefore the answer is the population in the north grew by 2.8%. I didn't know that the question asks that 2.8% is how much of 40%, which the answer is 7% (D).

Should the question be more specific, to avoid confusion? I think if the question asks for "percent increase" then it would be clearer. Thanks!

-C

You were close! And yeah, I understand your comment, but when all your answers are percents you have to assume that you are dealing with percents.

You found that the north grew from 40% to 42.8%. Remember, percent change is the difference over the original amount. Thus, the change of 2.8% divided by the original amount of 40% is like 28/400 or 7%.

I hope that this helped! :-)

_________________
Jamie Nelson
ManhattanGMAT Instructor