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← Can you solve the riddle and escape Hades? - Dan Finkel

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Showing Revision 3 created 10/09/2020 by lauren mcalpine .

  1. Maybe the fates got clumsy.
  2. Maybe Poseidon had one of his angry days.
  3. However it happened,
    the underworld is overcrowded,
  4. and Zeus has ordered Hades
    to let some spirits out.
  5. Hades arranges all the souls
    of the dead in a line before Cerberus.
  6. When one of his three heads bites
    down on the soul in front of it,
  7. they’ll get returned
    to the land of the living.
  8. Anyone to the left must get out
    of line and stay in Hades forever.
  9. And everyone else shuffles forward,
    at which point Cerberus will feed again.
  10. Each of the dog’s heads has an equal
    chance of being the one to bite each time,
  11. and no two ever bite simultaneously.
  12. Unfortunately, Hades’ minions forgot
    to tell you what was happening,

  13. and by the time you show up
    there are only 99 souls left in line.
  14. Hades looks furious and drawing attention
    to yourself won’t end well.
  15. But suddenly, time freezes,
    and Hermes steps out of the shadows.
  16. He tells you he can instantly put
    you into the line,
  17. and no one will realize what happened.
  18. But he’ll only grant his grace
    to someone clever enough
  19. to take full advantage of it.
  20. Choose the best place in line
    and he’ll give you the spot.
  21. Choose wrong, and he’ll leave you to rot.
  22. Which spot should you pick?

  23. Pause the video to figure it out yourself.

  24. Answer in 3

  25. Answer in 2

  26. Answer in 1

  27. It’s possible to calculate the exact
    probability of going free

  28. in all 100 spots.
  29. But there’s a much simpler path
    to the solution
  30. that requires surprisingly
    little calculation.
  31. Imagine being anywhere in line.

  32. Way up at the front, one of the three
    heads will pick someone at random,
  33. and you’ll move forward 1, 2, or 3 spaces.
  34. Since each is equally likely,
  35. your chance of survival
    from wherever you started
  36. is the average of the chances from
    each of the three spaces in front of you.
  37. And this is where you can
    find a huge shortcut.

  38. Averages must be on or between
    the extremes of what you’re averaging—
  39. they can never be higher
    than the highest value
  40. or lower than the lowest.
  41. So whatever your chances of survival
    are where you start,
  42. one of the three places in front
    of you is at least as good,
  43. and probably better.
  44. This observation is incredibly powerful.

  45. It means that wherever you are in line,
  46. it’d be wise to trade your place for one
    of the three spots in front of you.
  47. Let’s ignore which for now and think
    of them as a trio—
  48. this trio’s maximum value
    is better than this trio’s, and so on.
  49. Keep going and you’ll reach the front...
  50. These three spots must contain
    the extreme values—
  51. the best and worst probabilities—
  52. for the entire line.
  53. In other words,
    they’re all we need to consider.
  54. Place 1 is bad.

  55. Head one would save you, and the other
    two doom you forever.
  56. That’s just a 1 in 3 chance to escape.
  57. Place 2 is better: head two is great,

  58. head 3 is bad, and head 1 is ok
    in that it gives you another chance.
  59. But place 3 is best,
    because head 3 saves you

  60. while heads 1 and 2 both
    give you extra chances.
  61. If you did want to consider
    the exact probabilities,

  62. the odds of surviving in place 3
    are 16 out of 27, or close to 60%.
  63. The spots later in line tend to be very
    close to having a 50% chance of survival.
  64. Why 50%?
  65. Because every time Cerberus sends
    one soul up to be reborn,
  66. he leaves 0, 1, or 2 souls
    in the underworld.
  67. That averages out to one person
    staying for each one who gets freed.
  68. But you can beat those odds handily
    with what you now know.

  69. Hermes has places to be, and so do you.
  70. He rewards your insight by sneaking
    you into the third spot.
  71. And from there it’ll be just a short wait
    to learn your ultimate fate.