Decomposing and composing 100 from random numbers

General

Students practice decomposing and composing the number 100 from random numbers.

Relevance

It is useful to be able to quickly split 100 into two numbers. If you are saving for something special, it is useful to know how much more you need to save.

Introduction

Students complete the number line and need to skip count in 1s, 5s and 10s.

Development

Show students the example on the interactive whiteboard and ask them to think about which is more. Show that both amounts are 100 and so they are equal amounts. Explain that one 10 dollar bill is equal to ten 1 dollar bills. Ask where they see 100 together and make clear that you need to count all of the dollars before you know the amount. It is not always easy to see right away. Explain that it is easy to compose 100 with money. First we count to the ten by adding dollar bills. Then we skip count in 10s using ten dollar bills. Check that students can do this by themselves by asking them to drag money to the left and having them write down the answers. Show the example with eggs. They compose 100 by first looking at which number we already have. First count the eggs. Then count on to see how many are needed. Explain that you can compose 100 with help from the number line. First count to the nearest ten, and then skip count on to 100. Next students practice completing number bond charts without visual support. They decompose the numbers in their heads.

Check that students can decompose and compose 100 from random numbers by asking the following questions:
- What is a useful way to decompose 100? (first count to the nearest 10 and skip count in 10s)
- With 19, which number composes 100?

Guided Practice

Students practice decomposing 100 in a number bond chart with objects as visual support. Then they decompose 100 by selecting the numbers which compose 100 and finally they complete the number bond chart for 100 without visual support.

Closing

Discuss with students that it is important to be able to decompose 100 so you can figure out how much you need to save to reach 100 from a given amount. It can also be useful if you are working with money. Ask students to complete the decomposition of 100.