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?
Let R1 and R2 be the red balls, B be the blue ball, and W be the white ball. There are 6 possibilities.
R1, R2
R1, B
R1, W
R2, B
R2, W
B, W
5 of the possibilities have at least 1 red ball, and 1 possibility has both red balls. Therefore, the probability is 1/5.

Posted by Math Man
on 20170524 09:30:46 