I have some blue and red sock in my drawer.

I have total of 4 socks.

I pick 2 socks and chances that i get pair of red socks is 1/2.

What is the chance of picking a pair of blue socks ?

zero

I had 3 red socks and 1 blue sock.

=> chances of pair of red socks = 1/2

chances of pair of mixed socks = 1/2

chances of pair of red socks = NULL