 Subtraction to 20 without regrouping

# Subtraction to 20 without regrouping

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## General

Students learn to subtract numbers to 20 without regrouping tens.

1.OA.C

## Relevance

It is important to be able to subtract numbers to 20, so you can determine how many you have left. If you start with 17 blocks of cheese and then eat 6, you can determine that you have 11 blocks of cheese left.

## Introduction

The interactive whiteboard shows a number line showing a number of jumps. Ask students which number has been jumped to. Then show an empty number line and ask students to jump to a given number. Ask students how they determine how to do so and drag the jumps to the number line.

## Development

Discuss with students what the steps of subtraction are. First you count the total number of objects, this is the first number in your subtraction problem (the minuend). Then you count how many are taken away, this is the second number of your subtraction problem (the subtrahend). When you take the second away from the first you are left with the difference. Demonstrate this with the apples and chocolates on the interactive whiteboard. Check that students are able to do this by asking them to subtract the glasses and light bulbs. Ask students which subtraction problems represent the images shown. Tell students that they can use different strategies for subtraction. You could use a rekenrek or blocks, you can count back on a number line, or you could also see how many you need to add to the second number (subtrahend) to equal the first number (minuend). You can also just subtract at once in your head. Have students find the difference of the next subtraction problems. Ask them how they determined the difference. Finally show students how to solve a story problem. Make sure students know how to determine which numbers are needed for the math problem, what kind of math problem it is, to say or write the math problem, and finally to solve the math problem. Practice this as a class and then ask students to solve a story problem.

Check that students are able to subtract to 20 without regrouping by asking the following questions:
- What do you look at when you solve a story problem?
- How do you solve 18 - 6?

## Guided practice

Students first practice with subtracting two numbers to 20 with visual support. They then are given the subtraction problem without visual support and finally are given a story problem to solve.

## Closing

Discuss with students that it is important to be able to subtract numbers to 20 so they are able to calculate and determine how many objects they have left. On the interactive whiteboard, show the students an image and ask them which subtraction problem represents the given image. Ask them to write the difference on a sheet of paper. Then, show the subtraction problems and ask students to give the differences they found previously. You can drag the numbers on the board to where they belong.

## Teaching tips

Students who have difficulty with subtraction can be supported by the use of manipulatives like MAB blocks. Have them set out blocks to represent the total amount and then take away the second number and count the quantity that is left.

## Instruction materials

optional: MAB blocks or other manipulatives

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