Last weeks Brainteaser was obviously far too easy. So get the brain cells really fired up for this one…
Once again the US blog is competeing against the UK blog readers. We have timed the posts to go up at exactly the same time. I know the US will prevail!
Adam the t-shirt salesman has three business cards: a black card that is black on both sides, a white card that is white on both sides and a mixed card that is black on one side and white on the other. He pulls one from his pocket at random and hands it to you. The side facing you is black. What is the probability that the reverse side is also black?
a) 1/3
b) 1/2
c) 2/3
Lindsay Patross
Dave
I’m fairly sure it’s 2/3, because the black/black card has a greater probability of showing a black face than the black/white card, but I’d hate to have to write a proof.
I think the answer is c, too. There’s a 1/6 chance of you getting the WB card with W face up, and a 1/3 chance of you getting a BB with the B face up. So, if you turn it over, you’re twice as likely to get another black face than you are to get a white face (because you are twice as likely to have picked the BB card). 2:1=2/3. c! Thank you, probability tree (now, if I’m wrong I’ll be embarassed… but I haven’t done this in a fairly long time!).
b
I agree with Nekkid.