Add a multiple of 10 to any two-digit number within 100.

In this lesson, students add a multiple of 10 to any two-digit number, such as 64 + 30. While some students may initially apply their ability to mentally add 10 by counting on by tens (64, 74, 84, 94), students also decompose 64 into 60 and 4 to solve, as shown to the right.