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You are on an island where a volcano is going to erupt anytime soon, wiping everything with it. The only way to leave the island is via boat, but the captain of the boat asks 80 £ for the ride. You have only 70 £. In the island there's a casino with roulette. What do you do to maximize your chances of survival?

(riddle taken from a Colin Bruce book)

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    $\begingroup$ That boat captain is an a'hole. He'd let you DIE for the lack of a tenner? $\endgroup$ – Rand al'Thor Nov 21 '17 at 18:48
  • $\begingroup$ Stay along with the captain, sure it saves your life. ;-) $\endgroup$ – Mea Culpa Nay Nov 22 '17 at 18:22
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Place your bet where you have a 50% chance of winning (Red, Black, Odd, Even, etc.)

  1. Gamble £10. If you win £10, you're safe. If you lose, you still have £60 left.

  2. Gamble £20. If you win £20, you're safe. If you lose, you still have £40 left.

  3. Gamble £40. If you win £40, you're safe. If you lose, you're screwed.

So you have only a 1-in-8 chance of not making it off the island. Pretty good, given the circumstances.

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  • $\begingroup$ You were so fast in solving the riddle, I can't even accept your answer yet :) $\endgroup$ – dr01 Nov 21 '17 at 18:53
  • $\begingroup$ @dr01 Thanks for the edit. I'm not really familiar with how a roulette works :-) $\endgroup$ – Rand al'Thor Nov 21 '17 at 18:59
  • $\begingroup$ Relevant (and certainly long familiar to Rand, even if he doesn't know much about roulette specifically, since he's a mathematician): en.wikipedia.org/wiki/St._Petersburg_paradox $\endgroup$ – Gareth McCaughan Nov 21 '17 at 21:10
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    $\begingroup$ Incidentally, on actual roulette wheels you have slightly less than a 50% chance of winning when you make such bets, because the wheel has an extra space (or sometimes multiple extra spaces) that don't count as red or black, as odd or even, etc. This doesn't change the choice of strategy, but it makes it slightly less likely to succeed. $\endgroup$ – Gareth McCaughan Nov 21 '17 at 21:11
  • $\begingroup$ @GarethMcCaughan Perhaps even more relevant: en.wikipedia.org/wiki/Martingale_(betting_system) $\endgroup$ – GentlePurpleRain Nov 21 '17 at 21:58

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