Ahighway inspector needs an estimate of the mean weight of trucks crossing a bridge on the interstate highway system. she selects a random sample of 49 trucks and finds a mean of 15.8 tons with a sample standard deviation of 3.85 tons. the 90 percent confidence interval for the population mean is:

The confidence interval is


We first find the z-score for this confidence interval:
Convert 90% to a decimal:  90/100 = 0.9
Subtract from 1:  1-0.9 = 0.1
Divide by 2:  0.1/2 = 0.05
Subtract from 1:  1-0.05 = 0.95

Using a z-table (http://www.z-table.com) we see that the z-score for this value is directly between 1.64 and 1.65, so we will use 1.645.

The confidence interval is given by: