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5 dice, probability that a player achieves at least four 6’s in a round

Source: 2020 Dürer Math Competition Finals Day2 E+13 https://artofproblemsolving.com/community/c1622639_2020_

January 7, 2022
probabilitycombinatorics

Problem Statement

In the game of Yahtzee , players have to achieve various combinations of values with 55 dice. In a round, a player can roll the dice three times. At the second and third rolls, he can choose which dice to re-roll and which to keep. What is the probability that a player achieves at least four 66’s in a round, given that he plays with the optimal strategy to maximise this probability?
Writing the answer as p/qp/q where pp and qq are coprime, you should submit the sum of all prime factors of pp, counted with multiplicity. So for example if you obtained pq=3411255\frac{p}{q} = \frac{3^4 \cdot 11}{ 2^5 \cdot 5} then the submitted answer should be 43+11=234 \cdot 3 + 11 = 23.