Hi MoM,

As you continue to study, it's beneficial to 'build up' more than one set of skills. Most GMAT questions are designed so that they can be solved in more than one way, so it's valuable to redo questions using different methods (even if you correctly answered the question on the first try).

This question is built around Number Properties (which is a subject that is heavily represented in the Quant section - especially in DS). If you have the proper Number Property knowledge, then you can beat this question without having to do too much work (although you'll still want to take notes so that you can avoid making little mistakes). You can also solve it by TESTing VALUES.

We're told that A/B is POSITIVE. This means that both variables have the same 'sign' (they're either both positive OR both negative). Keeping both possibilities in mind is important because the question asks what MUST be true (which means "which is ALWAYS TRUE no matter how different examples you can come up with).

Knowing that the two variables COULD both be negative would allow you to immediately eliminate 3 answers: A, B and E. Taking that logic one step further, you probably already know that (positive)(positive) = positive and (negative)(negative) = positive, so one of the remaining two answers is clearly the correct one...

Final Answer: C

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

If the quotient a/b is positive, which of the following must be true?

(A) a>0
(B) b>0
(0 ab>0
(D) a-b>0
(E) a + b>0

We are given that the quotient a/b is positive (i.e., greater than zero). We know that in division when a quotient is positive, the two numbers being divided are either both negative or both positive.

This is also the case in multiplication; that is, to get a product that is positive, we must multiply two positive numbers or two negative numbers. Thus, the only answer choice that MUST be positive is C, ab > 0.

Answer: C