=IF(COUNT(C6:D6)=2,(C6+D6)/2,"")
=IF(COUNT(C6:D6)=2,IF(E6<65,65-E6,"-"),"")
=IF(COUNT(C6:D6)=2,IF(E665,E6-65,"-"),"")

Note, as Mike H pointed out earlier in the thread, that you didn't specify
what to do if E6 was exactly 65.
--
David Biddulph

Lithium wrote:
Now I'm really confused. Let me give some details.

C6 is the High temperature (no formula)
D6 is the Low temperature (no formula)
E6 is the Average Formula: =(C6+D6)/2 . This shows as 0 if C6 and
D6 are blank.
F6 is HDD Formula: =IF(E6<65,65-E6,"-") . This shows as 65 if C6
and D6 are blank.
G6 is CDD Formula: =IF(E665,E6-65,"-") . This shows as - if C6 and
D6 are blank.

If C6 _or_ D6 is blank then E6, F6 and G6 _should_ also be blank.

As I have said, if I have numbers for High and Low everything works
right. It's just that when the High and Low have not yet been
entered, the Average, Heating Degree Days and Cooling Degree Days
should be blank (or have just a dash).

TIA