GMAT Data Sufficiency Strategy - The Obvious Answer Trap

Data Sufficiency questions are supposed to be hard; more so than any other question type they tend to represent a chess match between you and the author, as the author has two chances to get you to make a mistake.  She won’t likely waste either statement giving you an easy pass – the questions have to elicit something from you in terms of efficiency or ingenuity in order to answer them correctly, so if an answer choice seems obvious within 15-20 seconds and you can’t spot a trap, well, you just fell into the trap.  Consider the question:

What is the value of x?

1) 3x + 2y = 15

2)  y = (-3/2) (x – 5)

This should pretty obviously be C. 

Two equations, two variables, neither works alone but both work together, right? 

But that is too easy, and the GMAT won’t often give you the answer that quickly.  Much as though the author had moved a pawn to expose her rook in a chess match to bait you into giving up your queen, you should take this situation to make sure that the author isn’t luring you into an easy trap. 

With that as your motivation to rearrange the algebra, you’ll notice that statement 2 is exactly the same relationship as statement 1:

y = (-3/2) (x-5)

2y = -3(x – 5)

2y = -3x + 15

2y + 3x = 15

While you may not see this right off the bat, the relative ease with which choice C comes to you should be your guide – don’t accept an easy answer without digging into the statements a little further to further investigate.