Can someone explain how to approach these types of problems

osirus0830 wrote: If x and y are positive integers, which of the following CANNOT be the greatest common divisor of 35x and 20y?
A) 5
B) 5(x – y)
C) 20x
D) 20y
E) 35x

Each is a multiple of 5, so GCD is 5 multiplied by GCD(7x, 4y)

A) is certainly possible, as 7x and 4y could have no prime factors in possible.

C) looks easier to think about than B), so skip B

For GCD to be 20x, GCD(7x,4y) must be 4x. But how can 4x be a factor of 7x? This looks promising! If I were running out of time, I'd choose C and move on!

D) For GCD to be 20y, GCD(7x,4y) must be 4y. This seems possible (for example if x = 4y)

E) For GCD to be 35x, GCD(7x, 4y) must be 7x. Case similar to D)



If I were pressed for time, I'd choose C now. If I had extra time, I'd revisit B

Note that if y and x are consecutive integers, 5(x-y) = 5. Look for 2 consecutive integers so that x is odd, for example :GCD (7(3), 4(2)) = 1

Thanks, that is how MGMAT approaches it as well, it is just time consuming, so I was wondering if there was a short cut I was unaware of.