 Dividing by a decimal number

# Dividing by a decimal number

Students learn to divide by a decimal number.

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## General

The students learn to divide a whole number or a decimal number by a decimal number with 1 or 2 decimal places.

## Standards

CCSS.Math.Content.5.NBT.B.7

## Learning objective

Students will be able to divide a whole number or a decimal number by a decimal number with 1 or 2 decimal places.

## Introduction

Show a sort of maze on the interactive whiteboard. The students follow a route. Each number that they come across must be subtracted from their total. In this way, they try to finish at the lowest number that they can. As soon as they come to a dead end, they are done with the assignment. Ask the students what their final number was.

## Instruction

First explain how you can divide a whole number by a decimal number with one decimal place. You can do this in different ways. Explain that you can do this by first changing the problem so that it does not contain a decimal. By multiplying both numbers in the problem by 10, the decimal point is solved out of the problem. After that you solve the problem without the decimal number. You can use a helping row to solve the problem. Explain that the problems with and without the decimal point have the same answer. Next you explain the other method of solving problems. First see how many times 0.4 fits in 1.6. This is 4 times. Then you know that 0.4 fits 40 times in 16, since both numbers are multiplied by 10. After this you have the students practice with several problems. Explain that you can divide a whole number by a decimal number with two decimal places in the same way. Have students also practice solving this type of problem. Then explain that for some problems for which both numbers are decimal numbers, you can also turn both numbers into whole numbers. In the case of numbers with two decimal places, you multiply both numbers by 100. Then you can solve the problem. Emphasize that the students can always use a helping row in their calculations. Have the students practice solving problems with decimal numbers with one and two decimal places. Next you explain how you solve a story problem and the students practice solving two additional story problems.

Check whether the students can divide a whole number or a decimal number by decimal numbers by asking the following questions:
- What steps do you take to solve a problem with decimal numbers?
- How do you solve the following problem: 91 ÷ 0.07?
- How do you solve the following problem: 14.1 ÷ 4.7?

## Quiz

The students test their understanding of dividing by a decimal number through ten exercises. In each of these exercises, they must solve a problem in which they must divide by a decimal number. Some of the exercises are story problems.

## Closing

You discuss with the students once again that it is importnat to be able to divide a whole number or a decimal number by a decimal number with one or two decimal places, so that you know how much everyone gets if you want to divide something evenly. Next you have the students solve the problems on the interactive whiteboard. Each answer that they solve forms a part of the code to the safe. If the students solve all of the problems correctly, they get the correct code to the safe. Next you divide the class into six teams. Each team takes turns throwing a die and takes that number of steps with their placeholder. Next that team solves the problem in the middle of the board. If their answer is wrong, they take a step back with their placeholder. If the problem is correct, they may stay where they are. Then it is the next team's turn. Keep going in this way, until one team reaches the end of the board. You can drag each of the problems on the cards off to the side to reveal the following problem. If you have already gone through all of the cards, then the team that is the farthest on the board wins.

## Teaching tips

Students that have difficulty with dividing by decimal numbers can first practice dividing larger numbers without decimal points. Next you show that you can also make this problem into one with a decimal point, for which the answer is the same (2.600 ÷ 2 is the same as 26 ÷ 0.02). For problems with two decimal numbers, explain that you move the decimal point one place to the right when you multiply by 10 and two places when you multiply by 100.

## Instruction materials

A die for the closing activity.

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