If you want every other column starting at C8 then your sequence is out of
order:
C8, E8, G8, I8, K8 M8, O8, Q8, R8, T8, V8, X8, Z8, AB8, AD8, AF8, AH8, AJ8
The order gets messed up after Q8.
Since you don't have a whole lot of cells:
=AVERAGE(C8,E8,G8,I8,K8,M8,O8,Q8,S8,U8,W8,Y8,AA8,AC8,AE8,AG8,AI8)
Or, if there are more cells you can try this array formula** :
=AVERAGE(IF(MOD(COLUMN(C8:AJ8)-COLUMN(C8),2)=0,C8:AJ8))
This formula will not *exclude* empty cells. To exclude empty cells:
=AVERAGE(IF(MOD(COLUMN(C8:AJ8)-COLUMN(C8),2)=0,IF(C8:AJ8<>"",C8:AJ8)))
** array formulas need to be entered using the key combination of
CTRL,SHIFT,ENTER (not just ENTER)