Management at the local Speedy Lube has found that it takes…

Management at the local Speedy Lube has found that it takes an average of 17 minutes, with a standard deviation of 2.5 minutes, in order to fully complete an oil change service. Management decides to take advantage of the quick service provided with a marketing campaign that promises “50% off your bill if we take longer than 20 minutes”.   After several months, management sees that they are giving out too many discounts, and they want to revamp the program.  They find that this program will be very profitable if they give the discount to no more than 3% of their customers.  If this is the case, how long should they make the guaranteed time limit?