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(2): simpler computation by Bob Genisot)

While this is true, you haven't demonstrated WHY *"by the time the 6th fly passes by, the spider has a probability of 0.5 of being 'hungry'"*. (Unless you're relying on the calculations of previous comment-posters.