Wednesday, March 4, 2009

Have I Got A Riddle For You (Revised)

I have three pennies. Without showing you the outcome, I flip them in succession. I tell you that the first two landed tails up. (Q1) What is the probability that the third one also landed tails up?
(A) 1/2
(B) 1/4
(C) 1/6
(D) 1/8

I flip them in succession again. Now I tell you that two of them landed tails up. (Note: I am only giving you information about two of the coins.) (Q2) What is the probability that the third penny I flipped also landed tails up?

When I said, "two of them landed tails up," I referred to two pennies. (Q3) What is the probability that the penny to which I haven't referred also landed tails up?

And finally, (Q4) what is the probability of at least one more tail if I tell you that one of pennies is tails up?

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