Hi,

The remainder when 120 is divided by single-digit integer m is positive, as is the remainder when 120 is divided by single-digit integer n. If m > n, what is the remainder when 120 is divided by m – n?

It is really necessary to pay attention and understand to the highlighted part in the question. As the remainder is positive it cannot be zero because, zero is neither positive nor negative.
Thus the number which gives us a remainder zero while dividing the number by 120 should be ignored.
i.e. m and n cannot be 1, 2, 3, 4, 5, 6 & 8 as all of them are divisible to 120
Leaving us with 7 & 9. And as m>n
m=9; n=7
m-n=2

Leaving us with remainder zero.

Hope this helps!