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

MATLAB: An Introduction with Applications
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ISBN:9781119256830
Author:Amos Gilat
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Chapter1: Starting With Matlab
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A company that runs a chain of quick oil change centers, tries to attract customers by claiming that it completes oil changes faster than its competition. So it periodically estimates the time taken to do oil changes at its centers. From historical records, the company knows that the population standard deviation of oil change duration is 15 minutes. Recently, 40 customers were asked how much time they spent getting an oil change at one of their centers. Based on the sample data, the chain concluded that the average oil change time for the population of all its customers was between 15 and 25 minutes. What was the confidence level at which the inference was made?
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