My approach to this question is...

Null Hypothesis: mu = 126.5ml

Alternative Hypothesis: mu is less than 126.5ml

critical value is 1.645 because 5% level of test and is a 1 sided test.

z =( 126.2 - 126.5) / (0.6 / sqrt 20) = -2.236

-2.236 is lesser than -1.645 (insignificant)

Therefore, there is insufficient evidence that the mean of manufacturing process is less than 126.5ml.

Is this correct? Advices on the approach please?