Barb wrote:
I apologize for this stupid question

There are no stupid questions :-).

but I'm trying to calculate the mean of
the following cells and format it into a percentage
[....]
Here's my formula:
=(B3*C3)+(D3*E3)+(F3*G3)+(H3*I3)+(J3*K3)+(L3*M3)/(C3+E3+G3+I3+K3+M3)

Not the best way to arrange the data. But to correct your formula, you
only need to insert some parentheses around the entire numerator, just
as you have them around the denominator, to wit:

=( (B3*C3)+(D3*E3)+(F3*G3)+(H3*I3)+(J3*K3)+(L3*M3) ) /
(C3+E3+G3+I3+K3+M3)