Popular, self-checking and fun—these three-corner flash cards are a great way to master addition and subtraction through 12! The unique, three-sided cards teach not just the operations, but also the relationship between them. Each side of a card features three numbers—kids add the two blue numbers…subtract a blue number from the purple number…and discover that the sum or difference is the third number on the card! Great for use at home or in the classroom and for small-group or individual practice, the set includes 46 durable, double-sided cards in a handy reusable container; each card measures 5 1/2".