In previous lessons, students learned that a teen number is composed of a ten and some ones and related addition and subtraction with teen numbers in equations. Students also learned that counting is related to addition, and they added numbers using the commutative property.

The purpose of this lesson is for students to solve addition story problems within 20 in which one addend is a teen number. Students then find the unknown value in addition equations where one addend is a teen number. Students may count all, count on, or recognize that they can add the ones and then add the ten. This lesson prepares students for future work, when they add within 100 using methods involving place value.

What aspects of today’s lesson allowed each of your students to see themselves as productive mathematical reasoners?