Topic E Addition of Two-Digit Numbers Now that students have experienced adding two-digit numbers to one-digit numbers and adding a multiple of 10 to a two-digit number, they are ready to add 2 two-digit numbers. Students leverage place value understanding to make problems easier. They use a variety of concrete, pictorial, and abstract tools to model addends as tens and ones. Students record their reasoning by using a written method and then explain their strategy. The goal of topic E is to build number sense that allows students to flexibly manipulate two-digit addends. At first, students self-select ways to combine groups of cubes that represent 2 two-digit numbers. They share how they decomposed each group and combined the resulting parts. Subsequent lessons present the three following ways to add 2 two-digit numbers: • Add like units: Decompose both addends into tens and ones, combine tens with tens and ones with ones, and then put tens and ones together. • Add tens first: Decompose one addend into tens and ones, combine the tens with the other addend, and then add the ones. • Make the next ten: Decompose one addend into tens and ones, combine some (or all) of the ones with the other addend (in many cases to make the next ten), and then add the remaining parts. Add Like Units
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Add Tens First
Make the Next Ten (Add Ones First)
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