 Subtracting numbers to 100 from a tens number

# Subtracting numbers to 100 from a tens number

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## General

Students learn to subtract numbers to 100 from a tens number.

2.NBT.B.5
2.OA.A.1

## Relevance

Discuss with students that it is important to be able to subtract numbers to 100, to determine how many you have left.

## Introduction

Give the class a starting number and have the students count forward by turns. At a given moment, you say "stop" and give students a second number to count backward from. Take care that each student gets a turn and switch between counting forward and backward as many times as you like.

## Development

Explain to students that tens are a group of ten and always end in a zero. This lesson is presented in a visual, abstract, and story method. You can use the blue menu in the bottom right to navigate between them and to select the method best suited to your class. In the visual, tell students that they can subtract by counting. First count how many there are in total, that is the first number of your subtraction problem (minuend). Then count how many are taken away, this is the second number of your subtraction problem (subtrahend). When you take the second number away from the first you are left with the difference. Show this on the interactive whiteboard using the chocolates and drumsticks. Check that students are able to subtract by counting by asking them to subtract the problems with glasses of lemonade and dots. Ask students to say or write the subtraction problem they just solved. Next the abstract method. Tell students that you can subtract numbers in different ways. One way is to subtract using a number line. You can also divide the second number into tens and ones to subtract it from the first. You can also subtract the second number from the first in one step. Emphasize to students that they must carefully look at the subtraction problem and decide how they want to subtract, but that different students might make different decisions. Ask students to solve the next set of problems individually or in pairs. Finally the story problems. Show students how to solve a story problem. They first determine what kind of math problem it represents, then they determine which numbers are important for the math problem. They next say or write the math problem and finally solve the problem. Do a story problem together as a class and then have students solve a story problem individually or in pairs.
Check that students are able to subtract numbers to 100 from a tens number by asking the following questions:
- What steps do you take to solve a story problem?
- How do you calculate 40 - 4 and 90 - 61?

## Guided practice

Students first practice with subtraction problems with visual support, then are given abstract problems, and finally they must solve story problems.

## Closing

Discuss with students that it is important to be able to subtract numbers to 100, so you can determine how much you have left. Show the money on the interactive whiteboard and ask the students to solve the problem. Repeat this with the problem with blocks. Ask students to explain their strategy. Next show students a set of problems with answers. Have students check to see if they are correct or incorrect. Erase the grey boxes to check.

## Teaching tips

Students who have difficulty subtracting numbers to 100 can be supported by making use of a number line. Remind them that they can take jumps larger than one when counting on a number line.

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