This is part of common core math

Making Tens (and Hundreds): Composing and decomposing: Students learn to break down numbers to make friendly numbers like 10 or 100, which are easier to add. Example: To add 8 + 5, they might see that 8 needs 2 to make 10. They could take 2 from the 5, leaving 3. Then, they add 8 + 2 = 10, and 10 + 3 = 13.

They are teaching new students this