Write numerals 1-3. Represent decompositions with materials, drawings, and equations, 3 = 2 + 1 and 3 = 1 + 2.

In this lesson, students continue to learn about the decomposition of numbers, extended from Topic C. Using linking cubes, they create a tower of 3, counting up as they add each cube. Then students break the tower into two towers, one of 1 cube and the other of 2 cubes. Students discover that all the towers together still make 3. They learn how to write a number sentence to represent the cubes. Finally, students move from the concrete to representing the numbers sentence with drawings and an equation.