The thickness of glass sheets produced by a certain process

The thickness of glass sheets produced by a certain process are normally distributed with a mean of ? = 3.25 millimeters and a standard deviation of ? = 0.12 millimeters.

What is the value of c for which there is a 99% probability that a glass sheet has a thickness within the interval [ 3.25

Solution

from a z- table (interpolating) z-score for a 99% confidence interval ?