There are four balls in a hat: a blue one, a white one, and two red ones. Now I draw simultaneously two balls, look at them, and announce that at least one of them is red.
What is the chance that the other is red as well?
The chance that they are both red = 1/6.
The chance that at least one is red = 1 - 1/6 = 5/6
1/6 divided by 5/6 = 1/5.