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A sample of 100 of a particular brand of tires found the mean life span to...

A sample of 100 of a particular brand of tires found the mean life span to be 50,000 miles . The population standard deviation is 800 miles . (a) Construct a 90% confidence interval containing the mean life span of all tires.
(b) How large must the sample be in order to be 90% confident that the mean lifespan of tires is within 500 miles of the sample mean ?
(c) How could we reduce the margin of error?
Math   Statistics-And-Probability   
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