Using SUMPRODUCT to find weighted averages
 

So I am trying to complete this project and I have a question with using SUMPRODUCT and dropping the lowest test score. This is the formula I am using:

=ROUND(SUMPRODUCT(B4:N4,$B$1:$N$1)/SUMPRODUCT($B$1:$N$1),0)

From B1:K1 there are different weights according to different assignments. From L1:N1 there are three exams worth 15% of my grade and I'm trying to drop the lowest one. I am completely stuck and cannot figure this.