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.