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S.S. Minnow SOS (Posted on 2020-09-02) Difficulty: 3 of 5
The coast guard station has received a distress call from the S.S. Minnow, sinking near one of two islands in opposite directions from the station. The call was interrupted by radio failure on the Minnow before the tourboat could specify which of the two islands it was near.

The station chief knows from experience with that tour operator that there is a 20% chance the boat is near island A and an 80% chance it's near island B. The station has 13 rescue boats, and, again from experience, it is known that each rescue boat has, independently of the other search boats, a 20% probability of finding a distressed boat if indeed a distressed boat is present, effecting a rescue.

How should the 13 boats be split between the two islands to maximize the probability that the people aboard the Minnow will be rescued? What is the probability that they will in fact be rescued if that optimal strategy is followed?

Part 2:

Suppose the coast guard station has 40 boats available but each one has only a 5% probability, independently, of finding a ship in trouble (given there is one in that location to be found). And further, there's only a 10% probability the boat is near island A, 90% of being near island B.

  Submitted by Charlie    
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Solution: (Hide)
Part 1:

The coast guard chief should send 3 boats to island A and the remaining 10 to island B, resulting in a probability of 81.17... %.

Part 2:

The chief should send all 40 boats to island B, resulting in a 78.4... % chance of a rescue.

ps=.2
pf=1-ps
n=13
for p1=.1 to .9 step .1
  p2=1-p1
  mx=0
  for n1=0 to n
   n2=n-n1
   pa=p1*(1-pf^n1)+p2*(1-pf^n2)
   if pa>mx Then
     mx=pa
     best=n1
   Endif
  Next
  Print n,ps,p1,best,mx
next

ps is probability of each rescue boat's independent success
pf is probability of each rescue boat's independent failure
p1 is probability of sinking boat being near island A
p2 is probability of sinking boat being near island B
pa is overall probability of rescue

The output of this and one with ps changed to .05 used for finding the solutions.

The problem was presented by Paul J. Nahin in his book, Duelling Idiots and Other Probability Puzzlers, 2000, Princeton University Press.

I highly recommend the book to anyone interested in probability puzzles, as well as his Digital Dice.

Comments: ( You must be logged in to post comments.)
  Subject Author Date
Possibly solutionLisa Reynolds2020-10-06 14:40:37
SolutionSolutionBrian Smith2020-09-04 10:13:23
Hopefully a Solutionhoodat2020-09-02 19:37:11
re: Possibly solutionJer2020-09-02 09:46:00
Possibly solutionJer2020-09-02 09:38:59
Soln (the Minnow might be lost)Steven Lord2020-09-02 09:38:52
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