Modular arithmetic is the mathematics of remainders. It shows up in every programming contest because it prevents overflow and keeps numbers manageable.
In this section, I will show you clock arithmetic, the mod operator, and the properties that make it work. You will see why problems ask for answers mod 10^9 + 7, and how to compute huge powers without overflow.
By the end, you will handle modular operations confidently and understand the patterns that appear in contest problems.