I have a bag which contains three coins. One of them is red on one side, blue on the other; another of them is red on one side and green on the other; and the third is blue on one side and green on the other.

I would like to lay them out on the table so that one of each color is facing up.

I reach into the bag and randomly select a coin. I flip it onto the table and it lands red side up (you cannot see what color is on the other side).

Not wanting to risk flipping another red side up, I reach into the bag and look for a coin with a green side, and place it green side up on the table (once again you have no idea what color is on the other side).

If I were to take the last coin out of the bag and flip it, what is the probability that it will land with a blue side facing up?

The possible third coin is either:
Blue & Red -or- Red & Green -or- Blue & Green

The combinations possible (with first color as side up) are:
Blue & Red
Red & Blue
Red & Green
Green & Red
Blue & Green
Green & Blue

Thus, P(Blue) = 2/6 = 1/3

Edited on May 24, 2006, 10:50 pm

Posted by Dej Mar on 2006-05-24 10:09:33