teacher needs help with an evaluation form

Hello,

I've tried to make a form in exell to make counting less time-taking.

Now I'm stuck with counting the avarages.

I have 7 rows, every row is one lesson. At the bottom it counts up the avarages from every lessson. No problem .

But, when a student have missed for example lesson 4, the output in lesson 4 is 0. How do a make that it doesn't count a lessen with no point in the avarages?

Sorry for my english :-).

Form is attached.