An inelegant solution would be if the total on each sheet is held in
cell b12
create a formula in each sheet (say cell c12) if(b120,1,0). this can
be entered in each sheet by selecting all the tabs at the same time and
typing it in. If the value is greater than zero it creates a 1 the total
of the 1s can be your denominator in the formula.

Your average can then become
=sum(sheet1:sheet17!b12)/sum(sheet1:sheet17!c12)

Obviously if your sheet tabs are named differently you can reference
them as in your average formula


--
Dav
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