My solution is very similar to that of
Bunuel, but since the reasoning is slightly different, I decided to post it.
Let x be the number of NEW books sold, and y be the number of USED books sold. Then, we'll have:
15x + 10y = 125
Analyzing this equation, you can notice that x MUST be an odd value for the equation to hold true; otherwise, the ending 5 digit of the 125 total sales would disappear. For example, if x=2, then 15x=30, and regardless of the y value, the last digit of total sales value would be 0. Not good.
So, our mission is to find the total profit number that will produce an EVEN value for x. Let the profit equation be: 5x + 2y = A (A=profit).
15x + 10y = 125 (divide the whole equation by -5, so we can eliminate y value, since we're not interested in it)
Sum the new total sales equation with the profit equation:
-3x -2y = -25
5x + 2y = A
This is our final equation: 2x = A - 25.
Quickly test the alternatives (replacing by A) to see which one yields an EVEN x (
EmpowerGMAT suggests that in "which of the following" questions, we start by testing either D or E, since those are more likely to be the correct answer). You will find that alternative E yields x=8, while all others yield ODD values for x.
Answer: E
It's not a 2-minutes question, but if you have time in the GMAT day maybe it's worth spending a little extra time on it