I dont need answers but I need only to know what I should fi

I don\'t need answers, but I need only to know what I should find out Z value or T value to answer the following:

The mean (M) output of the process A and B are 56 and 50 for 25 days:

          test H0: (MA- MB)=0, H1: (MA- B)<0, at alpha=0.05 ?

          (1) at standard deviation of (MA- MB)=20 test ?

          (2) at standard deviation of (MA- MB)=30 test ?

          (3) at standard deviation of (MA- MB)=15 test ?

(4) at standard deviation of (MA- MB)=40 test ?

Solution

Here you should use t-test becuase standard deviation of population or populations is unknown. We used z-test where standard deviaiton of population or populations are known.