gmat pep num props

[vinviper1]

Thanks!

[mav800rick]

1) List could be simply -3,-2,-1 OR 1,2,3

2) could be 2,-2 or 2,2

1) and 2) combined even if we have -4,-3,-2,-1 etc. Product of -4 and -1 must be positive.

[Ian Stewart]

From 1, the product of the largest and smallest is positive. This means the largest and smallest have the same sign: both are positive *or* both are negative. Well, if the largest and smallest are both positive, so is everything in-between. Similarly, if the largest and smallest are both negative, so is everything in-between. So from 1, we know everything in the list is positive, *or* everything in the list is negative. If everything's positive, the product is of course positive. But if everything is negative, you'll only get a positive product if you're multiplying an even number of terms. That's why you also need statement 2.