Probability of rolling three dice without getting a 6 I am having trouble understanding how you get $91/216$ as the answer to this question. 

say a die is rolled three times
what is the probability that at least one roll is 6?


probability of getting at least a six on rolling = 1- probability of getting  no  sixes.
the  probability of getting no sixes on one rolling is $\left(\frac{5}{6}\right) $
so, for  3 times rolling , we have that, the probability of getting no sixes is $\left(\frac{5}{6}\right)^3 = \frac{125}{216} $
so, the probability of getting  at least one six is $1- \left(\frac{5}{6}\right)^3 = \frac{91}{216}$
i.e your answer.
I think this could be helpful for you
