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The distribution of the operating life for batteries produced by a manufacturer is approximately normal with...

The distribution of the operating life for batteries produced by a manufacturer is approximately normal with standard deviation 3 hours. A random sample of 19 batteries has a mean operating life of 20 hours.

(a) Find a 90% conÖdence interval for the for the true mean life of the battery.

(b) Find the minimum number of batteries that should be sampled so that the length of the 90% conÖdence interval will not exceed 0.5.

Math   Statistics-And-Probability   
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