10 persons are sitting in a row. In how many ways we can select three of them if adjacent persons are not selected?

15 persons are sitting in a row. In how many ways we can select three of them if adjacent persons are not selected?

If 30 persons are sitting around a circle. In how many ways can 2 person out of them be selected so that they are not adjacent.

There are 10 different books in a shelf. In how many ways we can select atleast one of them ?

In how many ways 2 persons can be selected from 4 persons?