* Probability Bytes

[pranav.dass29]

I've come across this question which has totally stumped me. Can anybody provide a logical answer to this question?

Suppose that there are two coins. One of them is a usual coin with one side heads and the other tails. If this coin is flipped, there is an equal probability of ½ of the coin turning up head or tails. The second coin is special, in that it has heads on both sides. Obviously if this coin is flipped it will always come up heads. Now suppose that a person named Fred chooses one of the two coins at random and flips it 5 times. On flip number 5 the coin turns up heads. Calculate the probability that the coin that Fred flipped on try 5 is the usual coin or the special coin. In general, suppose that Fred flips a head on toss n. Write a general expression that give the probability that the coin on toss n was a usual coin. Also write a general expression that gives the probability that the coin was the special one.

[RonPurewal]

Hi,
This doesn't appear to be a GMAT-related problem. Yes, the title of this folder is "general math questions", but the posts still need to pertain reasonably directly to the GMAT.
If, as I assume, this is not a GMAT-related problem, then, while certainly interesting, it has zero value on this forum. Perhaps even negative value (by distracting people from stuff that they actually need).

If this problem actually is from a GMAT-related source (which I sincerely hope it isn't), please follow the forum rules, and post the entire problem with all answer choices and a citation of the original source.

If it's not, we'll kill the thread in the next week.

Thanks.