a. What is the probability that a laser fails before 6060 hours?  b. If 9 lasers are used in a product and they are assumed to fail  independently, find the standard deviation of the average lifetime of  these lasers. (unit: hours)   c. If 9 lasers are used in a product and they are assumed to fail independently, what is the probability that there are exactly 4 lasers failing before 6060 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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The lifetime of a semiconductor laser at a constant gower is normally distributed with a mean of 6950 hours and a standard deviation of 620 hours. 
 a. What is the probability that a laser fails before 6060 hours?
 b. If 9 lasers are used in a product and they are assumed to fail  independently, find the standard deviation of the average lifetime of  these lasers. (unit: hours) 
 c. If 9 lasers are used in a product and they are assumed to fail independently, what is the probability that there are exactly 4 lasers failing before 6060 hours?

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