Working on the same simple chart NBVC helped me on, but this is for AVERAGE instead of SUM and with an added variable.

I've come up with this, which works:
=IF(COUNTIF(B3:B14,">0"),(AVERAGE(B3:B14)))
Then needed to add; IF(B17,"N/A"). I tried using AND getting an error of too many arguments?

This also works to give me N/A but not the the AVERAGE I need if B17 is not N/A:
=IF(COUNTIF(B3:B14,">0"),IF(B17,"N/A"),(AVERAGE(B3:B14)))
Thanks for your help!