[Math] what is the probability of sum of five dice is being 14

combinatoricsdiscrete mathematics

Five fair six-sided dice are rolled. The probability that the sum of the result being 14 is ?

My solution : I started it with this following method

$$\sum_{d_{1}=1}^{6}\sum_{d_{2}=1}^{6}\sum_{d_{3}=1}^{6}\sum_{d_{4}=1}^{6}\sum_{d_{5}=1}^{6} \left[z\right]^{14}z^{d_{1}+d_{2}+d_{3}+d_{4}+d_{5}}$$

And I obtained 85/972 as the solution , Is that right ?

Best Answer

From stars and bars the number of $n$-tuples of natural numbers summing to $k$ is given by $\binom {k-1}{n-1} $

The number of 5-tuples of natural numbers summing to 14 is given by $\binom {13}4 = 715 $

some of these will contain a number greater than six. To get the number of 5-tuples that correspond to 5 rolls of a 6 sided die you need to subtract these from 715. It is fortunate that no 5-tuple summing to 14 can contain more than one element greater than 6.

  • those that contain a 7 must also have 4 other natural numbers summing to 7,
  • there are $5\binom 63$ of these
  • and $5\binom 53$ will contain an 8

Total 5-tuples summing to 14 for which no element is greater then 6 is given by

$$ N = \binom {13}4-5\binom 63 -5\binom 53 -5\binom 43 -5\binom 33$$ where the last term comes from the 5-tuples containing one 10 and four 4's

$$ = 715 - 100-50-20-5= 540$$

Each 5-tuple has a probability of $(\frac 16)^5=\frac 1{7776}$

so the probability of summing to 14 is $\frac{540}{7776}= \frac 5{72}$