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?
Out of the set (R1, R2, W, B) there are 6 possible draws of 1 pair:
(R1W,. R2W, R1B, R2B, R1R2, BW) .
The first five justify announcing: "at least one is red" , but only one out of those five consists of two red balls.
So the answer is 1/5 i.e, 20%.
Edited on August 4, 2017, 1:40 am