=AVERAGE(IF((MOD(COLUMN(C6:Y6)-COLUMN(C6),2)=0)*ISNUMBER(C6:Y6),C6:Y6))
I am not the best at explanations, but here it goes.....
The MOD(
number,divisor) function finds the remainder of a
number divided by a
divisor.
In the formula above...it divides the column numbers between column C and column Y (with column C becoming column #0 via the COLUMN(C6:Y6)-COLUMN(C6) part of the formula), by the divisor, 2 (which, in this case is your N value - i.e every 2nd column).
The MOD(COLUMN(C6:Y6)-COLUMN(C6),2) function churns out an array like {0,1,0,1,0,1,0,1...etc).
The IF() condition says if the Mod() result=0 then TRUE. The IF() condition further checks that the value in row 6 is actually numeric (no text, errors). If both conditions are TRUE, then the ultimate result is TRUE
For every TRUE, the formula extracts the corresponding value from row 6 and finally it averages them all out.
Hopefully this cleared it up for you.
To see the steps of the formula in action, go to TOOLS|FORMULA AUDITING|EVALUATE FORMULA and continue clicking EVALUATE to each step of the evaluation. It should help you to understand how the formula works.