averaging across 2 sheets

[vtach3743]

I have a timesheet that keeps a 13 and a 26 week running average of
weekly hours. I create a new sheet for each year. How can I keep the
averages running between the sheets. My current line for inside a sheet
is:

MROUND((SUM(F10:F22)/13),0.25)

I'm not sure how to modify it to use say F10 thru F17 on last years
sheet and use F18-F22 on the current sheet to get the 13 week average.

Any help would be appreciated.

Also I need to be able to have a cell not allow a value to be entered
if another cells value is less than or equal to 0.

 

[Peo Sjoblom]

=MROUND((SUM('Last Year'!F10:F17,F18:F22)/13),0.25)