Answer:
m + 15
Explanation:
Start at the end and work backwards to the beginning of the sentence. Most times these work like that. I'll start with "m", then I know that 15 more than "m" is m + 15. It would be the same thing to express that particular problem as 15 + m because addition is commutative. But subtraction is not.
"15 less than m" is m - 15, wheras
"m less than 15" is 15 - m.
This matters because if m = 10, for example, 10 - 15 = -5 where 15 - 10 = 5
5 and -5 are not the same answer.