you can approach this in two ways: by "brute force" programming(if program use java), and by pencil-on-paper analysis to come up with an analytic expression.

Eternal Dice Game Bob, Cob, and Dob are playing a game with a normal 6-sided die. All three keep playing rounds, over and over again, forever. In each round, Bob goes first, Cob goes second, and Dob goes third. The round is continued until someone finally rolls a 6, at which point that round ends. What is the probability that Dob is the first to roll a six