In[1]:= data = {553.`, 552.`, 567.`, 579.`, 550.`, 541.`, 537.`, 553.`, 552.`, 546.`, 538.`, 553.`, 581.`, 539.`, 529.`};

In[2]:= ZTest[data, 20, 540, "TestStatistic"]

Out[2]= 9.81495

In[3]:= myZTest[data_List, s_, m_] := (Mean[data] - m)/(s/Sqrt[Length[data]])

In[4]:= myZTest[data, 20, 540]

Out[4]:= 2.19469