Hi rsarashi,

Certain types of DS questions will appear on Test Day to measure the thoroughness of your thinking.

The prompt tells us about two ways that Ellen could purchase a computer:
1) From a store - for $P + 6% tax
2) From a catalog - for a total price of $Q

We're asked if it will cost Ellen MORE to buy the computer from the store than from the catalog. In real simple terms, it's asking "Is 1.06(P) > Q?" This is a YES/NO question.

Fact 1: Q - P < 50

From this, we don't know if Q or P is bigger, but we can still TEST VALUES to prove the inconsistency....

IF...
Q = 50
P = 1
1.06(1) is NOT > 50. The answer to the question is NO.

IF....
Q = 50
P = 53
1.06(53) IS > 50. The answer to the question is YES.
Fact 1 is INSUFFICIENT.

Fact 2: Q = 1150

This tells us nothing about the value of P, so we cannot figure out the value of 1.06(P).
Fact 2 is INSUFFICIENT

Combined, we know....
Q - P < 50
Q = 1150

At this point, many Test Takers would assume that this was insufficient, without gathering ANY proof. It's always interesting when Quant questions include specific numbers - those numbers were CHOSEN for a reason. They are NOT by accident. So it's interesting that Q = 1150 EXACTLY. Maybe there's a reason....maybe there's a pattern here....

Substituting in the value of Q, we have...

1150 - P < 50
1100 < P

Now this provides a lower limit to what P can be, but we MUST factor in the 6% tax too....

At the 'low end', P = just over $1100. With a 6% tax, (.06)(1100) = $66, so 1.06(P), at the minimum = a little more than 1100 + 66 = $1166. Every other possible price for P is GREATER than this. By extension, EVERY possible value of 1.06(P) is greater than Q.
Combined, SUFFICIENT.

Final Answer: C

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