Every day I process microfilm for 3 hours, Monday - Friday.  I am tracking
the time that this is done, and averaging the time at the end of the week in
a spreadsheet.  (I am using Excell 2003)  If I work on the microfilm 3 hours
for five days the average for the week is 3 hours - easy.  
However, the work is not steady and some days there is no microfilm to
process - So I may work 3 hours for 5 days one week, or for 4 days the next
week, or none at all!  I still want an average of 3 hours no matter the
number of days , because that is how long the average time to process the
microfilm is for each day.  And if the time ever changes - like we get new
equipment that is faster and cuts the time down to 2 hours, I would like that
reflected in the calculations also.

I hope that I am overlooking something simple, but everytime I try to wrap
my brain around this it seems to snow ball out of control!  Any suggestions?

Thanks in advance,
~Tim