The length of life of oil-drilling bits depends upon the types of rock and soil that the drill encounters, but it is estimated that the mean length of life is 55 hours. An oil exploration company purchases drill bits that have a length of life that is approximately normally distributed with mean equal to 55 hours and standard deviation equal to 9.1 hours.(make it easy to see)

a. What proportion of the company's drill bits will fail before 40 hours of use?

b.What proportion will last at most 65 hours?

c.What proportion will have to be replaced after more than 75 hours of use?

d.35% of all drill bits have a life less than how many hours?

e.What is probability of 2 randomly chosen drill bits last longer than 45 hours?

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