How to calculate average total working days for tasks completion

[Laras]

Hi,

I'm newbie, and I hope you guys can help me to solve this problem
I have a list of working files that contains the opening file dates, closing file dates, and also the status of working files. I have already calculate the working days of each of the working file using NETWORKDAYS function

=IF(OR(B2="",C2=""),"",IF(ISERROR(NETWORKDAYS(B2,C2)),"N/A",NETWORKDAYS(B2,C2,$I$2:$I$12)-1 & " days"))

and now I want to calculate the average of total working days excluding the blank cells and the error values but can't seem to find out how. I've tried this formula but it doesn't work either :

=AVERAGE(IF(ISNUMBER(E2:E11),E2:E11))

Please kindly help me to work this out
I've also attached the Example file to help you understand my questions better.
Thx before.