If a1 is mapped to 2 , we have ‌7C5 ways of mapping rest of the elements. If a1 is mapped to 3, we have ‌6C5 ways of mapping rest of the elements. If a1 is mapped to 4, we have ‌5C5 ways of mapping rest of the elements. Hence total number of increasing functions =‌7C5+‌6C5+‌5C5=28.