Step-by-step explanation:
There are 21 socks in total. Only 7 of them are white.
When you pick your 1st sock, you have a 7/21 chance to get white.
When you pick your 2nd sock, you have a 6/21 chance to get another white, since you already picked a white.
So the probability to get a pair of white socks is 2/21.
