Hi MY7MBA,

We're told that a box of balls originally contained 2 blue balls for every red ball and that after 12 red balls were added to the box, the ratio of red balls to blue balls became 5 to 2. We're asked for the TOTAL number of balls in the box BEFORE the additional 12 balls were added. This question can be solved by TESTing THE ANSWERS.

Let's TEST Answer B: 12 total balls
Since we have 2 blue balls for every 1 red ball, that means that 2/3 of the balls are blue.
Starting Blue = (2/3)(12) = 8
Starting Red = 12 - 8 = 4
Adding 12 red balls gives us....
Blue = 12
Red = 4 + 12 + 16
Red:Blue = 16:12 = 4:3
This does NOT match what we were told (the ratio is supposed to be 5:2). To increase this ratio, we need the 'impact' of adding 12 red balls to be greater - which means that we need FEWER red balls to start. There's only one answer that does that, but here's the proof that it's correct:

Let's TEST Answer A: 9 total balls
Since we have 2 blue balls for every 1 red ball, that means that 2/3 of the balls are blue.
Starting Blue = (2/3)(9) = 6
Starting Red = 9 - 6 = 3
Adding 12 red balls gives us....
Blue = 6
Red = 3 + 12 + 15
Red:Blue = 15:6 = 5:2
This is an exact match for what we were told, so this MUST be the answer.

Final Answer: A

GMAT assassins aren't born, they're made,
Rich