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a probabilities question - correction

Posted By: Tom Keith
Date: Monday, 2 July 2018, at 2:40 p.m.

In Response To: a probabilities question - correction (Boop)

What is the probability that, if I pick 2 random rolls from a match file, they will be the same? So there is a number of games variable G and a number of rolls in match variable R. In my match file G = 15 and R = 432.

I think* this is enough information to get an answer.

• Let a = total number of choices of two rolls from the file = C(432,2)
• Let b = number of choices of two rolls from the 15 opening rolls = C(15,2)
• Let c = number of choices of two rolls from the 417 nonopening rolls = C(417,2)
• Let d = number of choices of 1 opening roll and 1 nonopening roll = a - b - c

• Let pb = probability that two opening rolls match = 1/18
• Let pc = probability that two nonopening rolls match = (1/6)(1/36)+(5/6)(1/18)
• Let pd = probability that one opening and one nonopening roll match = (1/6)*0 + (5/6)(1/18)

The probability that two random rolls from that file will be the same is:

(b/a)*pb + (c/a)*pc + (d/a)*pd

(* If I got it wrong, my excuse is that I'm a programmer not a mathematician. What I really want to do is write a simulation.)

Messages In This Thread

• a probabilities question
  Boop -- Sunday, 1 July 2018, at 11:36 a.m.
  • a probabilities question
    Mike Clapsadle -- Sunday, 1 July 2018, at 12:57 p.m.
    • a probabilities question
      Bob Koca -- Sunday, 1 July 2018, at 4:39 p.m.
  • a probabilities question
    bluedice -- Sunday, 1 July 2018, at 5:35 p.m.
    • a probabilities question
      Mike Clapsadle -- Sunday, 1 July 2018, at 5:55 p.m.
      • a probabilities question
        Mike Clapsadle -- Sunday, 1 July 2018, at 5:58 p.m.
    • a probabilities question - correction
      bluedice -- Sunday, 1 July 2018, at 6:39 p.m.
      • a probabilities question - correction
        Mike Clapsadle -- Sunday, 1 July 2018, at 7:23 p.m.
        • a probabilities question - correction
          bluedice -- Monday, 2 July 2018, at 7:43 a.m.
      • a probabilities question - correction
        Bob Koca -- Sunday, 1 July 2018, at 8:09 p.m.
        • a probabilities question - correction
          bluedice -- Monday, 2 July 2018, at 7:56 a.m.
          • a probabilities question - correction
            Boop -- Monday, 2 July 2018, at 9:46 a.m.
            • a probabilities question - correction
              Bob Koca -- Monday, 2 July 2018, at 1:44 p.m.
            • a probabilities question - correction
              Tom Keith -- Monday, 2 July 2018, at 2:40 p.m.
              • a probabilities question - correction
                Bob Koca -- Monday, 2 July 2018, at 3:05 p.m.
                • a probabilities question - correction
                  Tom Keith -- Monday, 2 July 2018, at 5:40 p.m.
          • a probabilities question - correction
            Bob Koca -- Monday, 2 July 2018, at 1:40 p.m.
  • Two relevant puzzles
    Bob Koca -- Sunday, 1 July 2018, at 8:07 p.m.
    • Two relevant puzzles
      Mike Clapsadle -- Sunday, 1 July 2018, at 8:45 p.m.
      • Two relevant puzzles
        Bob Koca -- Sunday, 1 July 2018, at 8:50 p.m.
        • Two relevant puzzles
          Mike Clapsadle -- Sunday, 1 July 2018, at 8:59 p.m.
    • Two relevant puzzles-Puzzle 2
      roadkillbooks -- Sunday, 1 July 2018, at 10:26 p.m.
      • Two relevant puzzles-Puzzle 2
        Tom Keith -- Sunday, 1 July 2018, at 10:47 p.m.
        • Two relevant puzzles-Puzzle 2
          Bob Koca -- Sunday, 1 July 2018, at 11:08 p.m.
        • Two relevant puzzles-Puzzle 2
          roadkillbooks -- Monday, 2 July 2018, at 5:23 p.m.
          • Two relevant puzzles-Puzzle 2
            Bob Koca -- Monday, 2 July 2018, at 6:56 p.m.