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 Charlie)
Actually Charlie you were almost correct originally. If you look at the problem in terms of how many flies have made it past the spider rather than calculating how many have been eaten, you find 5C3=10 cases that have 3 have escaped, 5C4=5 that 4 have escaped and 5C5=1 that all 5 have escaped. These are the only cases in which the spider is still hungry, i.e. 16/32 = 1/2 of the time.
re(3): simpler computation
