You are interested in determining the average cost of a 3-minute telephone call to locations within the continental U.S.What sample size must you take to be 96% confident that the results will be within $.75 of the true mean cost per call? From the phone company,you have gotten an estimate of $\text { ? }$
= 7.71.
A) 185
B) 445
C) 406
D) 574
E) 22

Question

You are interested in determining the average cost of a 3-minute telephone call to locations within the continental U.S.What sample size must you take to be 96% confident that the results will be within $.75 of the true mean cost per call? From the phone company,you have gotten an estimate of $\text { ? }$
 = 7.71.
A) 185
B) 445
C) 406
D) 574
E) 22

Quizplus · Accepted Answer

We use the formula for sample size calculation for means:

n = [(z-value)^2 x σ^2] / E^2

where:
z-value = 2.05 (from z-table with 96% confidence level)
σ = 7.71
E = 0.75

Plugging in the values, we get:

n = [(2.05)^2 x (7.71)^2] / (0.75)^2
n = 444.9

Rounding up to the nearest whole number, we get a sample size of 445. Therefore, the best choice is B) 445.

You Are Interested in Determining the Average Cost of a 3-Minute