In this section, students add a two-digit number and a one- or two-digit number without composing a ten. They build on the work of prior units, in which they added one-digit numbers and teen numbers without composing a ten, and added multiples of 10 to two-digit numbers. In this section, students see that when adding two-digit numbers they can add...

In this section, students apply what they learned about adding one- and two-digit numbers to add any numbers within 100—with and without composing a ten. Students apply the associative and commutative properties as they count on, add tens and tens, and add ones and ones. Students see that no matter which order they use to combine parts of the addends,...

In this section, students learn that sometimes when adding two-digit numbers within 100, it is necessary to compose a ten from ones. They draw on the idea of making a ten, which they learned when adding ones and adding numbers within 20.

Students may not initially consider composing a new unit of ten when adding numbers. Students may count on...