Lesson 3 Relate Addition and Subtraction within 20

- Let’s find the number that makes equations with 20 true.

Warm-up Number Talk: Addition and Subtraction

Find the value of each expression mentally.

$7 + 3$
$10 - 7$
$10 - 2$
$10 - 4$

Activity 1 Introduce What’s Behind My Back: 20 Cubes

We are going to play What’s Behind My Back, this time with 20 cubes.

Activity 2 Make the Equation True

Find the number that makes each equation true.

$4 +$ $= 20$
$20 -$ $= 4$
$6 +$ $= 20$
$20 -$ $= 10$
$+ 3 = 20$
$20 - 15 =$
$20 -$ $= 18$
If you have time: $- 5 = 20$

Practice Problem

Problem 1

Find the number that makes each equation true. Show your thinking using drawings, numbers, or words.

$20 - 10 =$
$17 +$ $= 20$
$20 = 5 +$
$7 = 20 -$