



 
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 onedigit numbers that have a sum of 13, like 6 and 7 since
Then take one of these two numbers (I am going to use 6) and write it as the sum of two different onedigit numbers, like 4 and 2. Thus
I hope this helps,  


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