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?
Balls Drawn Chose to Fits known Is other
Balls announce situation red?
1234
BWRR 1 & 2 1
BWRR 1 & 3 1
BWRR 1 & 4 1
BWRR 2 & 3 2
BWRR 2 & 4 2
BWRR 3 & 4 3 Y Y
BWRR 1 & 2 2
BWRR 1 & 3 3 Y
BWRR 1 & 4 4 Y
BWRR 2 & 3 3 Y
BWRR 2 & 4 4 Y
BWRR 3 & 4 4 Y Y
2 out of the 6 equally likely possibilities, given that a red ball has been announced, have the other ball red as well, for a probability of 2/6 = 1/3.

Posted by Charlie
on 20170519 11:00:34 