Joanna,

I'll show you what I would do but for the situation where the sum is to be 13 rather than 15. I would look for two different one-digit numbers that have a sum of 13, like 6 and 7 since

13 = 7 + 6

Then take one of these two numbers (I am going to use 6) and write it as the sum of two different one-digit numbers, like 4 and 2. Thus

13 = 7 + 6 = 7 + 4 + 2

I hope this helps,
Penny