I have this formula in Sheet2:
=IF(OR(ISTEXT(L2),ISBLANK(L2)),"",IF(OR($M2>'Sheet1'!$A2+1,ISBLANK($M2)),NETWORKDAYS('Sheet1!$A$2,$L2),"")) to store the data in the W column.
And then I use disaply it in the sheet1 by using "=AVERAGE('Sheet2'!W3:W141)"

I intend to combine two formula together so I do not need to create many
columns in Sheet2 because the A colcumn in the Sheet1 increases.
I tried to use SumIf and Sumproduct, however, I have a hard time to make it
right.
Any suggestion will be highly appreciated.