Suppose that when a grocery store has 3 cashiers working during its afternoon rush period, a customer

Question:

Suppose that when a grocery store has 3 cashiers working during its afternoon rush period, a customer waits 8 minutes on average to be served.
If there are 2 cashiers working, what is the probability a customer waits more than 15 minutes in total if the customer has already been waiting 10 minutes?