There are $7776$ possible outcomes of rolling $5$ six-sided dice, of this total there are $651$ possible outcomes where the $5$ dice equal exactly $15$.

How could you calculate the number of possible outcomes where the top three dice equal $15$?

  • Careful enumeration of cases. It is all too easy to miscount. – André Nicolas Jul 31 '16 at 18:38
up vote 0 down vote accepted

There are three ways to roll 15 with three dice: 663, 654 and 555. I'll call this the greatest-sum-set (GSS); these have to be the greatest within the roll. I will split each case into three sub-cases, depending on how many times the smallest number in the GSS appears on the remaining two dice:

  • 663|xx (x is 1 or 2, the x's can be different). 10 ways to place 6's, 3 ways to place 3 afterwards, the remaining spaces can be filled in 4 ways. Total 10 × 3 × 4 = 120 ways.
  • 663|3x (one 3 on remaining dice). 10 ways to place 6's, 3 ways to place x, x can have 2 values. Total 10 × 3 × 2 = 60 ways.
  • 663|33 (two 3's). Only 10 ways to place 6's and no choice after that.
  • 654|xx (x < 4). 5 × 4 × 3 ways to place 654, remaining spaces can be filled in 9 ways. Total 540 ways.
  • 654|4x. 5 × 4 × 3 ways to place 65x, x can have 3 values. Total 180 ways.
  • 654|44. 5 × 4 ways to place 65, no choice afterwards. Total 20 ways.
  • 555|xx (x < 5). 10 ways to place x's, which can assume 16 values. Total 160 ways.
  • 555|5x. 5 ways to place x, which can assume 4 values. Total 20 ways.
  • 555|55. Only one way for this.

Adding up all cases gives 1111 5-dice rolls where the top three numbers sum to 15.

  • Thanks! Do you know if there is a formula for this? So as to use larger numbers. – danny Aug 1 '16 at 15:03
  • Theoretically a formula would be possible, but I would have to deal with so many cases that it would be unwieldy, especially since "number of ways to sum n whole numbers to N where no part is greater than d" is a partition function, examples of which are always terribly messy. It's usually better for these types of problems to define the cases on-site and count them individually. – Parcly Taxel Aug 1 '16 at 15:08

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