The May issue of Scientific American describes the

Sleeping Beauty Problem. On Sunday Sleeping Beauty is put to sleep and a coin is tossed. If it comes up heads she is awakened on Monday, asked what was the probability of heads, then is put back to sleep, which causes her to forget she was ever awakened.

If it comes up tails she is again awaked on Monday, asked what was the probability of heads, then put back to sleep forgetting she was ever awakened, then awakened again on Tuesday and asked the same question, then put back to sleep.

Whether the result of the coin toss was heads or tails, Sleeping Beauty is awakened on Wednesday.

What should Sleep Beauty have answered when awakened on Monday and Tuesday when asked about the results of the coin toss.

I'll give my answer in a later post. I don't know why this isn't trivially simple, but many mathematicians apparently think it isn't.

--Percy