This can be used as a center or as a review/race. I used it as a race in my class. I gave each group the problem cards (I color coordinated the cards). I then set out the answer cards at the front of the room. They had to solve the problems and then come to the front and find the answer card that matched. I had them place the problem card on the answer card. There are extra answer cards so they have to really pay attention to what they are doing. This is based on customary units of measurement.