Until this year the mean braking distance of a Nikton automobile moving at 60 miles per hour was 175 feet. Nikton engineers have developed what they consider a better braking system. They test the new brake system on a random sample of 64 cars and determine the sample mean braking distance (assume the population standard deviation is known to be 21 feet). How many cars should be tested if Nikton wants to be 95% confident of being within 4 feet of the population mean braking distance?