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?