Multiplicative Counting Principle

if a process is made of stages with separate choices, the total # of choices is $m\ast n\ast p\ast \dots$ for all choices. n - # of choices in first step m - # of choices in second step p - # of choices in third step

Hard question example

Athlete has 4 pairs of shoes. How many ways can he pull out 2 unmatched shoes 1 after the other?

Tree Diagram are a good way to represent these paths.