Adding mm and cm

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Students learn how they can add millimeters and centimeters together.

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

Students will be able to add millimeters (mm) and centimeters (cm) together.

On the interactive whiteboard there are a few numbers that the students must convert to millimeters or centimeters.

Ask if the students know how many millimeters there are in one centimeter. With help from a visual aid, show how you add centimeters and millimeters together. Explain that you must first convert the centimeters to millimeters, then you can add them. Also allow the students to try this themselves with help from a visual aid. Next, you explain that you can add centimeters and millimeters, by first converting centimeters to millimeters. To do this you take one jump to the right, so you multiply by 10. Discuss several problems with the class. Then the students must solve an addition problem together, for which they can choose from a few possible answers or give the answer themselves. Next, you explain that you can also convert to centimeters. You take one jump to the left, so you divide by 10. Have the students solve several problems on their own. Afterwards, you discuss addition of centimeters and millimeters in the form of a story problem. The story problem can be a multiplication or a division problem. Allow the students to solve a story problem on their own.

To check whether students can add millimeters and centimeters together, you can ask the following questions:

- How do you add centimeters and millimeters together, when you have to give the answer in millimeters?

- How do you add centimeters and millimeters together, when you have to give the answer in centimeters?

Students first practice adding millimeters and centimeters, for which they get a visual aid. Then they practice adding for which they are only given the numbers and they make a story problem, which they must then solve.

You discuss with the students again that it is important to be able to add millimeters and centimeters together, because that is how you can determine how much something is in total. Check if they know that they must first convert to the same length measurement by multiplying or dividing by 10. For this, use a few problems on the interactive whiteboard with paint splotches over the answers. Have the students solve the problems and drag the paint spot out of the way, to show what the correct answer is.

When students have difficulty with adding millimeters and centimeters, you can give them a diagram of the metric system, wherein you only have centimeters and millimeters with an arrow for ÷10 and an arrow for x10. Explain that you first have to convert. Then you can add the numbers together.

You may print out a diagram of the metric system.

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