Students explore pairs of numbers that make up a given number. Measuring 9" x 12", these plastic dry erase boards are printed on one side and blank on the other. Students see the inverse relationship between addition and subtraction when using number bonds. Knowledge of number bonds is essential when it comes to harder calculations involving addition and subtraction. These dry erase boards can be used in a variety of mathematical and verbal activities. Each set includes 10 boards. Sold as 2 sets for a total of 20 boards.