Please provide step by step solution to this:

A production line operation is designed to fill a container with a mean weight of 20 ounces. A quality control inspector selects a random sample of 100 containers to test whether overfilling or underfilling is occurring in the production line because if so, the line should be shut down and adjusted to obtain proper filling. The sample provided an average mean filling weight of 20.18 ounces. From past data, a standard deviation of 2 ounces is assumed for the population of filling weights. Let denote the mean filling weight in ounces for this production line.

What is the value of the appropriate test statistic to perform this test?