See the conflicting instructions only as they are responsible for the conflicts.
as the elements with a and c are independent we can arrange them in any order. so it will be (4! / 2*2). ---1
and now we can make the arrangement of the remaining B elements in 6 ways ( 4!/2*2) ----2 but only 2 are able to produce the conflict seriazable schedules. So it will be (4! / 2*2). *2 . ( from 1)
