Refer to Exercise 10.

Suppose that a 95% confidence interval is to be constructed for the mean time.

a. What is the critical value?

b. What is the margin of error?

c. Construct the 95% confidence interval.

Exercise 10

The amount of time that a certain cell phone will keep a charge is known to be normally distributed with standard deviation

σ = 16 hours. A sample of 40 cell phones had a mean time of 141 hours. Let μ represent the population mean time that a cell phone will keep a charge.