Excel 2007 : Subtotal

[Concerned]

im new to excel and my prof hasnt really explained how to do this. Im really struggling with it so any help would be appreciated. In column D21, i need to add the total assignment worth. I have marks for 2 assignments, so you place those in and the third one you dont, so you leave it blank. The values you add are determined by the subtotals, the highest subtotal is the scheme you use. So the answer should be 30. I always get 45, how do i fix this. Attached is the file.