The waiting time itself

Topic: Random variables and distributions · more questions on this topic
‹ The sum of two dice Distance to the centre ›

Two fair dice are thrown until the sum is 12, and X is the number of throws required. Give the probability mass function of X.

Worked answer (click to show)

\mathbb{P}(X=x)=(35/36)^{x-1}(1/36) for x=1,2,\ldots.