In how many ways 4 persons can occupy 10 chairs in a row ,if no two sit on adjacent chairs.
Given 4 persons and 10 chairs.
First place 10-4=6 chairs(C) leaving a gap(G) among each chair as follows:
As a result we have 7 gaps, these 7 gaps can filled with 4 persons as 7C4 ways and also multiplying by the number of ways to arrange the persons themselves to get required arrangements.
So, required number of ways=7C4(4!)=7!3!×4!(4!)
=7×6×5×4×3!3!×4!(4!)
=840 ways.