how to drop grades
 

This is what I have so far:
column E is for Quizzes and column I is for Labs.
Quizzes (col E) counts for 10%.
Labs (col I) counts for 20%.

I want to drop the lowest 4 grades out of all the labs and quizzes for each
student. For example, if the lowest 4 are all quizzes then no labs get
dropped.

Therefore to compute how much each grade is worth I need to divide the
percent by the number of undropped grades in each column.
MY QUESTION IS: how do you make this formula ?