Dividing the sum of two rows ignoring null entries

I'm trying to craft a gradesheet for my son's teacher and I am runnning into a problem obtaining a students average. In one row I've listed the possible questions for all the tests whether the test has been taken or not. In the row below I've listed "questions answered correctly" for only the tests they've taken. I need a way to calculate the overall avergae of the student while ingoring columns with null values in the "questions answered correctly" row. I know there is a way to ignore zeroes by excluding them with an IF statement but I didn't want to do that in case one of the students actually did get a zero. As the attached example shows the simple sum function renders a 25% average while the average should actually be 50%. Any help is appreciated.