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Testing Answers – B/D or C?

GMAC always puts numerical answers in ascending or descending order on the GMAT. Consequently, there is often an optimal approach to testing answers. Students often ask where they should start when testing answers. Many have heard C, and some have heard B or D, or just one of those. There definitely is a best way to do this…First, it’s important to realize that even though this strategy (testing your answers) will work for a lot of problems, it will not always be smooth. Some times you can tell whether an incorrect answer is too large or too small in the context of the problem. At that point there’s no way to take advantage of order of the answers.

That said, when the context of the problems is clear and you can determine whether an answer is too big or too small, starting with B or D is better than starting with C. To demonstrate this we’ll assume, for convenience, that our answers our ascending as we move from (A) to (E).  We’ll also assume that we will start with B. We could make the same point with descending answers and starting with D – or any variation on this setup.

(A) 1          (B) 2          (C) 3          (D) 4          (E) 5

If we start with B, there’s a 20% chance that this answer is correct. If it’s not correct, there’s a 50% chance that it’s too large (I’m assuming that the probability B is too large is equal to the probability that B is too small). If that’s the case then A must be the answer. If that’s not the case we have to check D. So –

  • B is correct 20% of the time, and incorrect 80% of the time.
  • A is correct based on B 50% of 80% (i.e. 40%) of the time.
  • If A can’t be the correct answer based on B, we have to check D, and based on D, we’ll know the answer

Now, what if we started with C.

  • C is incorrect 80% of the time
  • If it’s too big we have to check A or B to determine the correct answers
  • If it’s too small we have to check C or D to determine the correct answer.

Using the “Check C” means that 80% of the time you’ll have to check two answers. In the “Check B” (or D) scenario you only have to check two answers 60% of the time – there is a 40% chance that you can find the correct answer by only checking B.