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Spider and Fly (Posted on 2003-11-25) Difficulty: 3 of 5
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?

See The Solution Submitted by Ravi Raja    
Rating: 3.6250 (8 votes)

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re(2): simpler computation | Comment 8 of 21 |
(In reply to re: simpler computation by SilverKnight)

You are right of course that the spider would never actually eat another fly after having eaten 3. However, the spider's satiety is filled anyway, and for the purposes of the probabilities for fly #6, it's "as if" he had, and makes it less worrisome about missing cases.

In other words, consider the cases to be combinations that either were eaten or would have been eaten except for having been given the satiety reprieve.
Edited on November 25, 2003, 10:38 am
  Posted by Charlie on 2003-11-25 10:36:38

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