Solve add to with change unknown problems, relating varied addition and subtraction strategies.

In this lesson, students use "counting on," "take from ten" strategies, or the "get to ten" strategy to solve "add to with change unknown" problems. (For example, "In the morning, there were 8 leaves on the floor under the ficus tree. During the day, more leaves fell on the floor. Now, there are 13 leaves on the floor.") The "get to ten" strategy has students solving 13 - 8 as 13 - 3 - 5, understanding that decomposing the subtrahend to easily get to the ten yields a simpler, more manageable subtraction problem.