If you roll two dice, the probability that you roll a sum of 10 is approximately 0.083. What is the probability that you first roll a ten on the 10th roll? What is the average number of rolls until you first roll a 10?
Probability of rolling a 10 = 0.083 so probability of not rolling a 10
So probability of first rolling a 10 on the tenth roll = probability of not rolling a 10 nine times * probability of rolling a 10.