The flashlight batteries produced by one of the northern manufacturers are known to have an average life of 60 hours with a standard deviation of 4 hours. Use Chebyshev's theorem to answer the following questions.
a. At least what percentage of flashlights will have a life of 54 to 66 hours?
b. At least what percentage of the batteries will have a life of 52 to 68 hours?
c. Determine an interval for the batteries' lives that will be true for at least 80% of the batteries. (Hint: First compute the Z-score.)