First, two of the six positions are selected for Project A and Project B, and then two of the remaining four positions are selected for Project C and Project D, and the C (46 6,2) selection method is used. Since project B can only be carried out after the completion of project A and project D must be carried out immediately after the completion of project C, choosing a good location is equivalent to arranging the remaining two projects. Therefore, the different numbers of these six items are c (6,2 2) * C (4 4,2 2) * A (2 2,2) =15× 6× 2 =180.