F87:

=SUMIF(G$10:G$81,ROW(A1),I$10:I$81)

and copy down


ROW(A1) is just a shortcut to typing the number 1, which will adjust itself as the formula is copied down to 2, 3, 4, etc for each week. This will sum Column I for each instance of 1, 2, 3 etc down the column.