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for the following question:

the lifetime of a particular brand of cutting blade has a mean of 40 hours and sd of 8 hrs. when one blade fails, it is immediately replaced by a new identical blade. suppose there are 36 blades on hand. use CLT to find approximate probabilities:
a. what is the probability that sample average of lifetimes of the 36 blades exceeds 60 hrs?
b. compute the probability that all these 36 blades will be used up in less than 1250 hrs.

let n=36 and sd = standard deviation. can someone please explain why

a follows N(u, sd^2/n) and b follows N(n*u, n*sd^2)

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