We can think of it as four drawers:

White, red, green and blue

Suppose you have to draw at least x socks.

The problem is equivalent to putting x socks in four drawers, and there are at least 20 socks in the same drawer;

∴x÷4= 19……… 1

x= 19×4+ 1

x=76+ 1

x=77

Pigeon hole principle: put m objects into n drawers, and there must be at most [M/N] objects in a drawer, when m is not divisible by n; Take [m/n]+ 1]

[m/n] indicates the maximum integer value not greater than m n.

Answer finished, hope to adopt.