A particular brand of calc. battery is approx. normally distributed with a mean of 80 hrs and a standard deviation of 9 hours.

a) What is the probability that a single battery randomly selected from the population will have a life between 70 & 90 hrs?

P(70<x<90) = ?

b)What is the probability that 4 batteries from the population will have a sample mean life of between 70 & 90 hrs?

P(70<x<90)= ?

c) Reduce the standard deviation from 9 to 7 hrs. 4 randomly selected batteries

a90-70= 20%

b s=4

s= 10/ sqrt 4=5

c

need more time

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