The life in hours of a 100-watt light bulb is known to be approximately normally distributed with standard deviation ? = 25 hours. A random sample of 20 bulbs has a mean life of 1014 hours.
a. Construct a 95% two-sided confidence interval on the mean life
b. Construct a 95% lower confidence interval on the mean life
c. Suppose we wanted the total width of the two-sided confidence interval on the mean life of the bulbs to be 6 hours at 95% confidence, what sample size should be used?