Hi All,
This question can certainly be solved using a 2-variable "system" of equations. Since the prompt asks for B's CURRENT population, and the answers are numbers, we can TEST THE ANSWERS.
We're told that:
1) A's population is 400 greater than B's population.
2) If B's population were reduced by 900, then A's population would be 3(B's population).
We're asked for B's CURRENT population.
Let's TEST Answer B....
IF....
B's current population is 1650
A's current population is 2050
B - 900 = 1650 - 900 = 750
3(750) = 2250.....which is TOO BIG (in this scenario, it's supposed to be 2050).
This means that there should be FEWER people in both locations....
Knowing this, you can now pick the correct answer. Here's the proof though:
IF...
B's current population is 1550
A's current population is 1950
B - 900 = 1550 - 900 = 650
3(650) = 1950...which is a MATCH for what we were told.
Final Answer:
GMAT assassins aren't born, they're made,
Rich