A spider eats 3 flies a day. Until the spider fills his quota a fly has a 50% chance of survival if he attempts to pass the web.

Assuming 5 flies have already made the attempt to pass, what is the probability that the 6th fly will survive the attempt?

(In reply to

re: simpler computation by SilverKnight)

Thus, by the time the 6th fly passes by, the spider has a probability of 0.5 of being "hungry", so the 6th fly will be eaten with a prob. of (0.5)(0.5) = 0.25. The probability the 6th fly will survive is 1 - 0.25 = 0.75.