Two-Digit Number that Certain Sum and Difference (K-1st Graders Only)

The K-1st Grade classes explored two-digit numbers that had a sum of a certain number and a difference of a certain number. This challenge causes them to think about addition and substraction at the same time and strategically look for patterns.

First, I asked the children to give me several two digit numbers. We then looked at the sum of those digits and the difference. This proved to be a very easy exploration for them. For example, the number 51 has a sum of 6 and a difference of 4. When we say difference, we mean positive difference. So the number 15 has the same sum and difference.

Second, I only told them the sum and difference and they had to come up with the number. For example, if a number had a sum of 10 and difference of 2, they would have to first think of which two digits had a sum of 10: 9 and 1, 8 and 2, 7 and 3, 6 and 4, and 5 and 5. Which one of these has a difference of 2? Of course, only 64 and 46 would work.

The attached worksheet and solution pages were given to them to work in teams to solve these patterns.