I'm thinking that those would just be numbers between 0 and 9 that when you add them to 15 you wouldn't need to carry one number from one column to the other. If I am right, then the numbers that would work would be 0, 1, 2, 3, and 4. When you add 5 or a number greater than 5, you would have to carry over a "1" from the ones place to the tens place. A "1" is carried over when the sum of the numbers in the ones column is greater than or equal to 10. This is why the numbers 5, 6, 7, 8, and 9 wouldn't work.
* I am not a math expert. My little sister is in the second grade and she told me thats what she would do.
