A Traffic Engineer Plans to Estimate the Average Number of Cars

Multiple Choice

A traffic engineer plans to estimate the average number of cars that pass through an intersection each day. Based on previous studies the standard deviation is believed to be 52 cars. She wants to estimate the mean to within ±10 cars with 90 percent confidence. The needed sample size for n is:

A) n = 104 days.
B) n = 74 days.
C) n = 10 days.
D) n = 9 days.