Question

Suppose that a customer service center claims that the telephone calls it receives last 114 seconds on average, with a standard deviation of 9 seconds. If you took a sample of 324 telephone calls, which of the following mean times would be within the 95% confidence interval?

A. 116.5 seconds
B. 115.5 seconds
C. 114.5 seconds
D. 117.5 seconds

Answer 1

The 95% confidence interval is
\[\left(114-1.96\sqrt{\frac{81}{324}},~114+1.96\sqrt{\frac{81}{324}}\right)\]
or
\[(113.02,~114.98)\]