How to create a Formula item in the grade book that drops 2 of the lowest quiz scores

Kay.H.645

Hello,

An instructor wants to add a Formula item that:

(1) Drops the two lowest quiz scores from Category A (Note: There are seven quizzes in Category A)

(2) Averages the "Category A subtotal" and "Category B subtotal"

I would greatly appreciate any guidance you can give me.