Subtracting m and km

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

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

Students will be able to subtract meters (m) and kilometers (km) from each other.

There are several numbers on the interactive whiteboard which the students must convert to another length measurement. Have the students convert these and drag the covers away to check the answer.

Ask if the students know how many meters there are in one kilometer. Say that you can subtract meters and kilometers. Explain that you must first convert the kilometers to meters, and then you can subtract. To do this you take three jumps to the right, so you multiply by 1,000. Discuss several problems as a class. Then the students must solve a subtraction problem, for which they can choose from a few possible answers or give the answer on their own. Next, you say that you can also convert to kilometers. You take three jumps to the left, so you divide by 1,000. Have the students solve a few problems on their own. Sometimes you are not left with a rounded number, but you also have a few meters left over. First see how many kilometers you have and then how many meters. Discuss a few problems as a class, for which you must first convert the kilometers to meters and then you can solve the problem. For these problems it is asked how many kilometers and meters you have. The answer that you have found in kilometers must be then divided by 1000, to know how many kilometers and how many meters it is. Afterwards you discuss subtraction with kilometers and meters by means of a story problem. Have the students solve a story problem on their own.

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

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

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

Students first practice subtracting meters and kilometers with and without possible answers provided. Then they make a story problem, which they then must solve.

You discuss again with the students that it is important to be able to subtract with meters and kilometers, 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 1,000. Have the students practice with a few problems on their own. Behind each problem is a letter. Have the students solve the problems and place the letters by the matching answers. The correct answer is: if you have everything correct then... 'raise your hand!'

When students have difficulty with subtracting meters and kilometers, you can give them a diagram of the metric system, on which you only have meters and kilometers with an arrow for ÷1,000 and an arrow for x1,000. Explain that you must first convert. Then you can subtract the numbers from each other.

You may print out a diagram of the metric system.

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