Well, you aren't going to be able to do this with just the Average summary function. You're going to need to use a couple formulas with global variables to pull it off.
I'm not sure exactly what your raw data looks like. That is going to be important for determining how to structure the formulas. I'm going to offer up two scenarios.
If your raw data looks like the line you posted there, then you can simply create one formula to do the work. It would look something like:
Local NumberVar CountPO := 0
IF {MyReport.Jan} > 0 THEN CountPO = CountPO + 1;
IF {MyReport.Feb} > 0 THEN CountPO = CountPO + 1;
IF {MyReport.Mac} > 0 THEN CountPO = CountPO + 1;
IF {MyReport.Apr} > 0 THEN CountPO = CountPO + 1;
IF {MyReport.May} > 0 THEN CountPO = CountPO + 1;
IF {MyReport.Jun} > 0 THEN CountPO = CountPO + 1;
IF {MyReport.July} > 0 THEN CountPO = CountPO + 1;
IF CountPO > 0 AND {MyReport.Area} > 0 THEN
({MyReport.Total PO}/CountPO)/{MyReport.Area}
ELSE
0
If your raw data is more normalized, and looks like:
LotNo Date Area Month PO
1-0-1 OCT-07 1917 Jan 800
1-0-1 OCT-07 1917 Feb 0
1-0-1 OCT-07 1917 Mac 900
1-0-1 OCT-07 1917 Apr 0
1-0-1 OCT-07 1917 May 1000
1-0-1 OCT-07 1917 Jun 0
1-0-1 OCT-07 1917 July 0
Then you would need two formulas. In the details section, create a formula that looks like:
Global NumberVar CountPO
IF {MyReport.PO} > 0 THEN CountPO = CountPO + 1
In the footer section (either group footer or report footer, depending on your report), put this formula:
Global NumberVar CountPO
IF CountPO > 0 AND {MyReport.Area} > 0 THEN
(SUM({MyReport.PO})/CountPO)/{MyReport.Area}
ELSE
0
(If you are breaking this down by group, then you would need to use SUM({MyReport.PO},{MyReport.MyGroup}) instead.)
If neither of those solutions fits your needs, please feel free to clarify what your data looks like.