It is estimated that the mean life span of oildrilling bits

It is estimated that the mean life span of oil-drilling bits is 76 hours. Suppose an oil exploration company purchases drill bits that have a life span that is approximately normally distributed with a mean equal to 76 hours and a standard deviation equal to 12 hours. (Round your answers to four decimal places.)(a) What proportion of the company\'s drill bits will fail before 61 hours of use?
(b) What proportion will last at least 61 hours?
(c) What proportion will have to be replaced after more than 91 hours of use?

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