Algorithms Dead Episode 1: Division Under Mod
Do you blindly do operations under mod without knowing why they work? Does it scare you when a problem asks you do print something mod a big number? If so, you should watch Episode 1 of Algorithms Dead!
In it, I talk about why mod operations (addition/subtraction/multiplication) are allowed, why mod inverses work, and what things are safe to do when you store fractions as $$$p*q^{-1}$$$.