If they're integers, that means they can't be fractions (such as 15/2 and 2).
So the only couples of integers that, when multiplied, result in 15 are:
1 and 15
5 and 3.
(there are no other possibilities as both 5 and 3 are not divisible further).
The smaller of the two sums (15+1; 5+3) is 5+3=8.
The least possible sum of a and b is 8.