What factors to they have in common?
First, they're all even, so 2
if we divide all of them by 2, we get:
13,24,31
now, 13,31 are primes: so that means that those numbers don't have any more common factors!
So the greatest Common Factor of those 3 numbers is 2.
1/2x + 5 = 35
You use 1/2 because she sold half of her books . Then +5 because she gained 5 back and 35 because your end total should be 35.
Hope this helps ! :)