Subtracting cm and m

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Students learn how they can subtract centimeters and meters from each other.

CCSS.Math.Content.4.MD.A.2

Students will be able to subtract centimeters (cm) and meters (m) from each other.

On the interactive whiteboard there are several numbers that the students must try to convert to another length measurement within the time given.

Ask the students how many centimeters there are in one meter. Show with help from a visual aid how you can subtract meters and centimeters from each other. Explain that you must first convert meters to centimeters, then you can subtract them from each other. Have the students practice subtracting on their own with help from a visual aid. Then you explain that you can subtract meters and centimeters from each other by first converting meters to centimeters. To do this you take two jumps to the right, so you multiply by 100. Discuss several problems with the class. Next the students must solve a subtraction problem, for which they can choose from a few possible answers or give the correct answer by themselves. Sometimes you do not have a rounded number, but you have a few centimeters left over. First determine how many meters you have and then how many centimeters are left. Discuss several problems with the class, for which you must first convert the meters to centimeters and then you can solve the subtraction problem. These questions ask for the number of meters and centimeters that you have. The answer that you have calculated in centimeters must be divided by 100, in order to know how many meters and how many centimeters it is. Next you discuss subtraction of meters and centimeters through a story problem. Have the students solve a story problem on their own.

To check whether students can subtract centimeters and meters, you can ask the following questions:

- How do you subtract meters and centimeters from each other, when you must give the answer in centimeters?

- How do you subtract meters and centimeters from each other, when you must give the answer in meters?

Students first practice subtracting centimeters and meters, for which they get a visual aid. Then they practice subtracting with just the numbers and then they solve a story problem, for which they must subtract meters and centimeters from each other.

You discuss again with the students that it is important to be able to subtract with centimeters and meters, because that is how you can determine how much you have left of something. Check if they know that they must first convert to the same length measurement by multiplying or dividing by 100. Have the students practice this through a maze on the interactive whiteboard. The students must solve the problems and choose from the possible answers that are on the board. Then they follow that route and solve all the problems that they encounter along the way. If the students have all the problems correct, then they finish at a green check mark. If they have made a mistake, they reach a red X.

When students have difficulty subtracting centimeters and meters, you can give them a diagram of the metric system, in which you only have centimeters and meters with an arrow for ÷100 and an arrow for x100. Explain that you must first convert. Then you can subtract the numbers from one another.

You may use a printed diagram of the metric system.

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