You discuss once again with the students that it is important to be able to double, because that is how you can calculate the answer to a problem more easily. On the whiteboard there are three numbers that the students must double. By removing the covers, they can see the answers. Next you can give the students cards with decimal numbers, on which pairs of cards match each other because one is the double of the other. The students walk around through the class and look for the person with the double of their number, or whose number they have the double of.