A company produces batteries whose lifetimes are normally distributed with a mean life of 100 hours. It is known that 90% of the batteries last at least 40 hours. Estimate the standard deviation lifetime. What percentage of batteries will not last 70 hours?

A First Course in Probability (10th Edition)
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ISBN:9780134753119
Author:Sheldon Ross
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Chapter1: Combinatorial Analysis
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Problem 1.1P: a. How many different 7-place license plates are possible if the first 2 places are for letters and...
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A company produces batteries whose lifetimes are normally distributed with a mean life of 100 hours. It is known that 90% of the batteries last at least 40 hours.

  1. Estimate the standard deviation lifetime.
  2. What percentage of batteries will not last 70 hours?               
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