Which of the following CANNOT be weighed using a balance scale and the

While this question is not particularly difficult to answer, there are, at least, two things that we can learn from it.

The first is the value of considering ALL of the answer choices to a question. We may believe that we have found the answer to a question like this one when we have checked only the first few choices, but we may have missed something.

In this case, a test-taker who has checked the first few answers could decide that (A) is correct because there is no way to arrive at 13 by adding any of the numbers listed.

Of course that test-taker would have missed the fact that, by putting 1 on the same side as something weighing 13, 13 could be weighed.

If, however, even after getting the impression that 13 is the correct answer, the test-taker were to have considered the rest of the choices, the test-taker would have seen that 19 also does not work, and realize that he or she must have missed something.

So, in answering a question like this one, or a verbal question, considering all the choices can be valuable, and save us from missing a totally gettable question.

The second thing that this question highlights is the value of thinking out of the box. As soon as you see that 13 and 19 both seem impossible, it's time to get out of the box and find something that you have yet to see.

In doing so, you find that 13 works if you put 1 on the side of the scale where the 13 is.

Considering all possibilities and thinking out of the box are two key aspects of GMAT success.