I'm teaching a course that requires students to choose 2 of 6 units for a
value of 20% towards their course. My Excel sheet is set up so I can enter
the marks manually by units chosen, and each assignment is out of 100.
However, individual units do not have the same total points value.

How can I add up the points for each of the 2 optional units out of the
possible 6 and then calculate a value of 20%? Cells of units not chosen would
be left blank. Much appreciated!!

I'm not sure how units, assignments, marks differ in meaning, but it
seems you'd make a table on another sheet that holds the different
values for the units, then use vlookup to pull the correct amount.