Find the number of ways of seating 4 people on 11 chairs such that no two people are sitting on adjacent chair?

This ques can also be seen as "we have to arrange 4 people and 7 empty chairs such that no adjacent chairs are filled".
Take 7 empty chairs first, we know that there are 8 places between and at the ends of these empty chairs- which is to be filled by 4 people, which can be done in 8C4 ways i.e 70 ways.