A company that runs a chain of quick oil change centers, tries to attract customersby claiming that it completes oil changes faster than its competition. So it periodically estimatesthe time taken to do oil changes at its centers. From historical records, the company knows thatthe population standard deviation of oil change duration is 15 minutes. Recently, 40 customerswere asked how much time they spent getting an oil change at one of their centers. Based on thesample data, the chain concluded that the average oil change time for the population of all itscustomers was between 15 and 25 minutes. What was the confidence level at which the inferencewas made?