mandyrhtdm wrote:

vinnik wrote:

Which of the following CANNOT be a product of two distinct positive integers a and b?

A) a

B) b

C) 3b + 2a

D) b - a

E) ba

Answer is

Please help me in understanding this question. I am confused between C and D.

Thanks & Regards

Vinni

In these questions it is best to take an example as if something is true for all positive integers than it as to be true for the smallest and the easiest integers to work with

take a = 1 and b = 2 and work with the options

ab = 2

A) a take a =2, b = 1

B) b take a = 1 b = 2

C) 3b + 2a Seems tricky, lets see other options and then come back to it.

D) b - a take b = 1 and a = 2 --> b - a = -1 .. How the hell can product of two positive integers be negative ?? or less than each of them?

E) ba Always true

You don't even have to worry what C is !

Choosing a specific value for

b for which there is no

a which fulfills the condition is not a proof. There are infinitely many other possible values for

b. The statement is CANNOT be true, not that it MUST be true for any

a and

b.You have to prove/justify that there is no pair

(a,b) of positive integers for which the equality holds.

_________________

PhD in Applied Mathematics

Love GMAT Quant questions and running.