Talk Time on a Cell Phone Suppose Motorola wishes to estimate the mean "talk time" for its Timeport P8160 digital phone before the battery must be recharged. In a random sample of 40 phones, the sample mean talk time was 315 minutes with a standard deviation of 24 minutes.
(a) Why can we say that the sampling distribution of x is approximately normal?
(b) Construct a 94% confidence interval for the mean "talk time" for all Timeport P8160 digital phones.
(c) Construct a 98% confidence interval for the mean "talk time" for all Timeport P8160 digital phones.