This set of flash cards are addition only with a sum up to 20. They can be used many ways such as a center, sent home for extra practice, and even as a scoot game. In my class these are attached to construction paper and laminated for durability. This makes them easy to use with white board markers. You could also write the answer on the back of the card before laminating.
If you like to change things in your class with the seasons like I do, please check my store for more flash cards in different styles.