Not sure of what formula to use or the best way to do this

Hi;

I am trying to automatically give an average over several work sheets (tabs) but only if there is a value in the cell.  So if I have 10 sheets and I have a value in a specific cell on 4 sheets then I want to get an average for those 4 sheets only.  If I add a value to another sheet then it will be the average over 5 sheets.  But it won't be the average over all 10 sheets unless there is a value in all 10.

An example would be that in a cell on the sheets I have values of 2, 4, 6, 0, 0, 0, 0, 0, 0, 0.  I want to see the average be 4 ((2+4+6)/3) and not 1.2 ((2+4+6)/10).  And if I added the value of 8 onto another sheet the average would automatically become 5 and not 2.

Is there a formula for this type of thing and how would I apply it.

Thanks