The probability of rolling at most three sixes in 5 independent casts of this die

probabilityprobability distributions

A die is loaded in such a way that the probability of the face with j dots
turning up is proportional to j2 for j = 1, 2, 3, 4, 5, 6. What is the probability
of rolling at most three sixes in 5 independent casts of this die?

I have solved for $p(j)$:

$p(j)=1/91*j^2$

I don't know how to proceed further? I think binomial distribution will be used to solve it, but I don't know how to proceed. Any hint or help?

Best Answer

Note that the probability of getting a six is $\frac{6^2}{91}=\frac{36}{91}$. If $X$ is the number of sixes in the 5 independent casts, then $X\sim B\left(5,\frac{36}{91}\right)$. The probability you want can be found by $$\mathbb P(0\le X\le 3)=\sum_{k=0}^3\binom{5}{k}\left(\frac{36}{91}\right)^k\left(1-\frac{36}{91}\right)^{5-k}\approx 0.9163$$