The following problem is from "Probability and Statistics in Engineering - Hines, Montgomery" A potential customer enters an automobile dealership every hour. The probability of a salesperson concluding a transaction is 0.10. She is determined to keep working until she has sold three cars. What is the probability that she will have to work exactly 8 hours? More than 8 hours? Soln: I have done it. Don't know whether the process is correct. Also I think the answer is low in value. Want to get your opinion. She will have to work exactly 8 hours means - She will sell the last car at 8th hour and stop working. The probability of selling a car at 8th hour (any hour) = 0.1. Thus we have got the last hour calculated. We have the preceding 7 hours. Two cars will be sold at any two of the 7 hours and for remaining 5 hours no car will be sold. Probability of that = 7C2 . (0.1)2 . (0.9)5 = 0.124 Total Probability = 0.124 . 0.1 (for last hour) = 0.0124 I am trying the 2nd part.