PS question from GMAT prep

[missionGMAT007]

Hello,
Just try out this question from gmatprep.

Though I got it wrong during the exam and I got it correct while reviewing. I tried the same question once again to know how much time i would take and took 2.59 minutes even though i know all the steps. How can i solve these kind of questions faster?

[Geva@EconomistGMAT]

Hello,
Just try out this question from gmatprep.

Though I got it wrong during the exam and I got it correct while reviewing. I tried the same question once again to know how much time i would take and took 2.59 minutes even though i know all the steps. How can i solve these kind of questions faster?

One way is to focus only on the change between 2A (40,000) and a B (50,000)

A's down payment: 8,000 (20% of 40k) - 32,000 remaining

B's down payment 10,000 (20% of 50k) - 40,000 remaining.

So there's an 8,000 dollar difference in the remainders. Tack on 40% finance on the difference only: 40% of 8000 is 3,200, so the difference between the 2As and the B will have grown to 8,000+3,200 =11,200 by the time we pay the finance costs.
Now you just need to remember to tack on the 2,000 difference in the initial down payment....final answer is 13,200.

I have to say that you won't save much in this way - you'll make less calculations, but you'll need some time coming up with the strategy itself. It's a tough question, especially with that nasty 11,200 trap.