|Instead of manually changing your blank cells to zero, you could use a Helper Column.|
A helper column is a "intermediate" column between the actual data and the final result.
For example, with your real values, including blank cells in Columns A, drag this formula down Column B (or any other column, which you could then hide)
=IF(A1="", 0, A1)
Now use Mike's formula against the Helper Column, since there are no blank cells in that column.
Note: If I understand your requirements, you are always dividing by 15, so the COUNT function is probably not needed.
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message edited by DerbyDad03