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The New Coffee Room

  1. TNCR
  2. General Discussion
  3. Puzzle time: the runt of random points

Puzzle time: the runt of random points

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  • jon-nycJ Offline
    jon-nycJ Offline
    jon-nyc
    wrote on last edited by
    #1

    Choose 10 numbers uniformly at random from the unit interval [0,1].

    On average, what is the value of their minimum?

    "You never know what worse luck your bad luck has saved you from."
    -Cormac McCarthy

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    • HoraceH Offline
      HoraceH Offline
      Horace
      wrote on last edited by Horace
      #2

      :::

      I suppose the number where there's a 50/50 shot of never randomly choosing below it in 10 tries.

      :::

      Education is extremely important.

      jon-nycJ 1 Reply Last reply
      • HoraceH Horace

        :::

        I suppose the number where there's a 50/50 shot of never randomly choosing below it in 10 tries.

        :::

        jon-nycJ Offline
        jon-nycJ Offline
        jon-nyc
        wrote on last edited by
        #3

        @horace And that number would be?

        "You never know what worse luck your bad luck has saved you from."
        -Cormac McCarthy

        1 Reply Last reply
        • jon-nycJ Offline
          jon-nycJ Offline
          jon-nyc
          wrote on last edited by
          #4

          Hint:

          :::

          Think circle, not line.

          :::

          "You never know what worse luck your bad luck has saved you from."
          -Cormac McCarthy

          HoraceH 1 Reply Last reply
          • HoraceH Offline
            HoraceH Offline
            Horace
            wrote on last edited by
            #5

            :::

            .5=(1-n)^10

            =0.066967

            :::

            Education is extremely important.

            1 Reply Last reply
            • jon-nycJ jon-nyc

              Hint:

              :::

              Think circle, not line.

              :::

              HoraceH Offline
              HoraceH Offline
              Horace
              wrote on last edited by
              #6

              @jon-nyc said in Puzzle time: the runt of random points:

              Hint:

              :::

              Think circle, not line.

              :::

              Right, my formula would give the median but not the mean. Higher minimums have more room for outliers than lower minimums, which can only go to zero.

              Education is extremely important.

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              • KlausK Offline
                KlausK Offline
                Klaus
                wrote on last edited by
                #7

                :::

                If you choose 1 number, the average of the mimimum is 0.5.
                With 2 numbers, it's 0.333. With 3 it's 0.25.

                With n numbers it's 1/(n+1), hence for 10 it's 0.909...
                :::

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                • jon-nycJ Offline
                  jon-nycJ Offline
                  jon-nyc
                  wrote on last edited by jon-nyc
                  #8

                  Klaus is right.

                  Well, that was my answer. Official solution comes Saturday.

                  :::

                  One way to think about it is as follows: Pick 11 random points on a unit circumference circle (expected distance between points is 1/11). Randomly pick one as marking start/end point of unit line.

                  :::

                  "You never know what worse luck your bad luck has saved you from."
                  -Cormac McCarthy

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                  • HoraceH Offline
                    HoraceH Offline
                    Horace
                    wrote on last edited by
                    #9

                    I'm sure that's right.

                    Education is extremely important.

                    1 Reply Last reply
                    • jon-nycJ Offline
                      jon-nycJ Offline
                      jon-nyc
                      wrote on last edited by
                      #10

                      @Horace My original intuition was similar to yours but with a different formula.

                      The probabilities would be additive, not multiplicative, no?

                      So if m is the expected minimum value, that meant that:

                      10 * p(x<m) = 0.5, or
                      p(x<m) = 0.05
                      which gave me 1/20.

                      But as you pointed out that would be a median value, the mean would be higher than that.

                      "You never know what worse luck your bad luck has saved you from."
                      -Cormac McCarthy

                      HoraceH 1 Reply Last reply
                      • jon-nycJ jon-nyc

                        @Horace My original intuition was similar to yours but with a different formula.

                        The probabilities would be additive, not multiplicative, no?

                        So if m is the expected minimum value, that meant that:

                        10 * p(x<m) = 0.5, or
                        p(x<m) = 0.05
                        which gave me 1/20.

                        But as you pointed out that would be a median value, the mean would be higher than that.

                        HoraceH Offline
                        HoraceH Offline
                        Horace
                        wrote on last edited by Horace
                        #11

                        @jon-nyc said in Puzzle time: the runt of random points:

                        @Horace My original intuition was similar to yours but with a different formula.

                        The probabilities would be additive, not multiplicative, no?

                        So if m is the expected minimum value, that meant that:

                        10 * p(x<m) = 0.5, or
                        p(x<m) = 0.05
                        which gave me 1/20.

                        But as you pointed out that would be a median value, the mean would be higher than that.

                        My formula gives the correct median. It's the chance of rolling higher than "x" 10 times in a row, so multiplicative.

                        Education is extremely important.

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