You would need to use a workaround since array formulas do not work over
multiple sheets
=SUM(First:Last!K78)/SUMPRODUCT(COUNTIF(INDIRECT("'"&A2:A8&"'!K78"),"<>0"))
if there is no naming system like sheet1, sheet2 and so on then it is easier
to put the sheet names
in a range like the above where the sheet names would be in the A2:A8 range,
If you use Sheet2:Sheet4
with a naming system then you can use
=SUM(Sheet2:Sheet4!K78)/SUMPRODUCT(COUNTIF(INDIRECT("'Sheet"&ROW(INDIRECT("2
:4"))&"'!K78"),"<>0"))