Sums to 100 – Elementary Math

Purpose

To identify the missing addend in a sum

Materials

None

Overview

Give students a number and have them tell you how much must be added to it to make 100. For example, you might say, One hundred is 60 plus___? and let a student complete your statement (40). Repeat with more examples. You might want to pair problems with related answers so that students can use the first answer in each pair to find or check the second.

About the sequence

Part 1 starts with a number that is a multiple of 10, which students use that find the related fact. Part 2 starts with a multiple or 5, which students use to find the related fact. The extension introduce more examples of equations with missing addends that equal 100, using other non-decade numbers.

Part 1

Let’s find pairs that equal 100. For example, one hundred equals 60 plus __ (40). Let’s see if we can find the missing addends!

Examples:

One hundred equals 20 plus… (80)
One hundred equals 21 plus… (79)
One hundred equals 90 plus… (10)
One hundred equals 89 plus… (11)
One hundred equals 30 plus… (70)
One hundred equals 29 plus… (69)

When children are enjoying their building of mastery, feel free to repeat. When children are eager for more, try Part 2.

Part 2

Let’s find more number pairs that equal 100.

Examples:

One hundred equals 65 plus… (35)
One hundred equals 66 plus… (34)
One hundred equals 35 plus… (65)
One hundred equals 34 plus… (66)
One hundred equals 85 plus… (15)
One hundred equals 86 plus… (14)

As always, when children seem excited for a new challenge, move on.

Extension

Let’s keep practicing and find the missing number that gets us to 100.

Examples:

One hundred equals 51 plus… (49)
One hundred equals 87 plus… (13)
One hundred equals 76 plus… (24)
One hundred equals 49 plus… (51)
One hundred equals 33 plus… (67)