Modular arithmetic

Modular arithmetic is like telling time with a 12-hour clock. Instead of hours, minutes, and seconds, we use numbers. In modular arithmetic, we use the same number over and over again, we call this the "modulus" (for example, the modulus is 12 on a 12-hour clock). When we use modular arithmetic, we count from 0 up to the modulus, then start over at 0. So if the modulus is 12, we would count 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, then start over at 0. This pattern just keeps repeating forever. Modular arithmetic can be a helpful way to solve tricky math problems.