Ar Bs Ec Ar Bs Ec
B A C B B

I am trying to count entries for the data above. The 1st 3 columns are equal
to 1 entry each, and the last 3 columns are equal to half an entry. I want to
count the total no of entries. So for the above the result would be 2.5.
Column 4 and 6 would be discounted because column 1 and 3 have a grade in
there.

Not sure how to start, would be grateful for help.

Thanks