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 75 hours. An oil exploration company purchases drill bits whose length of life is approximately normally distributed with mean 75 hours and standard deviation 12 hours. What proportion of the company's drill bits
a will fail before 60 hours of use?
b will last at least 60 hours?
c will have to be replaced after more than 90 hours of use?