 Subtraction to 20 with one more or one less

# Subtraction to 20 with one more or one less

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## General

Students learn to subtract numbers to 20 by adding one more or taking one away.

1.OA.C

## Relevance

It is useful to be able to subtract with one more or one less. If you know this you can calculate sets of subtraction problems without having to recalculate each time.

## Introduction

Ask students to take turns counting. Start with the number given on the interactive whiteboard. They must first count forward. As soon as you say "stop" students must start counting backward. Restart but have students count backward and when you say "stop" they must start counting forward. Next show students a number line. Ask them which numbers are missing. How do they know which numbers are missing? Erase the grey boxes to reveal the missing numbers.

## Development

Show students the bananas on the interactive whiteboard. Count the bananas to determine that there are 15 bananas and that 6 have been eaten. That means that there are 9 bananas left. Discuss the other images and subtraction problems shown by the chocolates. Ask students if they notice anything about the subtraction problems. The first or second number is one more each time. It is not always possible to have images to support math problems, sometimes you only see the numbers. You then compare the subtraction problems. If the first number in the second problem is one more than the first number in the first problem, then the difference is one more in the second problem. Show the subtraction problems on the interactive whiteboard and ask students if they can determine the answers. Also ask them the steps they took to solve the second and third subtraction problems. Did they do the calculation again, or did they just take one away from the difference? Say that when the second number in the second problem is one more than the second number in the first problem, then the difference is one less in the second problem. Practice subtraction problems demonstrating this.
Next, show the bananas and determine as a class that there are 13 bananas. If four are eaten, that means there are 9 bananas left. Discuss the other images and subtraction problems shown by the bananas. Ask students if they notice anything about the subtraction problems. The first or second number is one less each time. It is not always possible to have images to support math problems, sometimes you only see the numbers. You then compare the subtraction problems. If the first number in the second problem is one less than the first number in the first problem, then the difference is one less in the second problem. Show the subtraction problems on the interactive whiteboard and ask students if they can determine the answers. Also ask them which steps they took to solve the second and third subtraction problems. Did they do the calculation again or did they just add one to the difference? Say that when the second number in the second problem is one less than the second number in the first problem then the difference is one more. Practice subtraction problems demonstrating this. Finally show subtraction problems on the interactive whiteboard and ask students to find the difference. Have them write their answers on a sheet of paper and hold them up to be checked as a class.

Check that students are able to subtract numbers to 20 with one more or one less by asking the following questions:
- What do you look at to know if you need to subtract with one more or one less?
- Calculate the following: 18 - 6 = and 18 - 7 =
- Calculate the following: 13 - 3 = and 14 - 3 =

## Guided practice

Students are first given multiple choice answers to select the correct answer from. Then they must fill in the correct answer for subtracting with one more and one less. Finally they must solve sets of problems with one more or one less.
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!---Closing
Discuss with students that it is important to be able to subtract to 20 with one more or one less, so you can easily and quickly solve sets of problems without needing to recalculate for each subtraction problem. To close the lesson, there are a series of subtraction problems with students providing answers. The class must decide which student has the correct answer. Ask students how they determined the correct answer.

## Teaching tips

Students who have difficulty with this learning goal can make use of MAB blocks or other manipulatives. Have them first set out the total amount of blocks. Then take away the number of blocks given in the subtraction problem. Then show a subtraction problem with one more. Show how this does not need to be recalculated, you can simply add one block.

## Instruction materials

optional: MAB blocks or other manipulatives

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