The desired amount of a chemical component X in a certain ty

The desired amount of a chemical component (X) in a certain type of aluminous cement is 5.5. To test whether the true average amount is 5.5 for a production facility, 16 independent samples are analyzed. Suppose that the amount of component X is a sample normally distributed with = 0.3 and that the sample mean is 5.25.

No other information is given.

What is the absolute value of the threshold of the rejection region?