Suppose cracks exist in a certain material and the crack len

Suppose cracks exist in a certain material, and the crack length [in micrometers (mum)] of a randomly selected crack is normally distributed with an average length of 71 nm and a variance of 6.25 (mum)^2. What percentage of cracks are over 74 mum long? Determine the 90-percentile value of the crack length, i.e., the crack length which has a 90% probability of not being exceeded. What percentage of cracks exceeding 72 mum are over 77 mum?

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