A bowling alley has 10 lanes. During rush hour, the average inter-arrival time between groups of customers is 2 minutes, with a standard deviation of 1 minute. Each lane can handle one group at a time. The average time taken by a group at a lane is 18 minutes, with a standard deviation of 9 minutes. What is the average time a group of customers wait for a lane to free up, after arriving at the alley