E.g.

A1=23, A2=14, A3=50 etc.

B1=12, B2=10, B3=32 etc.

So, the first mark is 12 out of 23, the second is 10 out of 14 and so on. I have a large number of these for at least 250 people. Is there a simple formula that will immediately work out the average percentage for all of the marks above? If so, will it still work if some marks are missing?