Until this year the mean braking distance of a Yamaha motorcycle moving at 65 mp

Question

Until this year the mean braking distance of a Yamaha motorcycle moving at 65 mph was 155 feet. Yamaha engineers have developed what they consider a better braking system. They test the new breaking system on a random sample of 25 motorcycles and determine the sample mean to be 145 feet. Since there is no previous information on population standard deviation, the standard deviation from the sample of 25 motorcycles was 28 feet.

How many motorcycles should be tested if Yamashita wants to be 95% confident of being within 5 feet of the population mean braking distance?

Explanation / Answer

alpha / 2 = 0.025 Z= 1.96

n = ( Z*sigma / error)^2

n = (1.96 * 28 / 5) ^2

n = 120.47

n = 121

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