20 persons are invited for a party. In how many different ways can they and the host be seated at circular table, if the two particular persona are to be seated on either side of the host?

 There are total 20 + 1=21 persons. The two particular persons and the host be taken as one unit so that these
remains  21 -3+1=19 persons be arranged in round table in 18! ways, But the two persons on either sides of
the host can themselves be arranged in 2! ways.

Required number of ways = 2! x 18! = $2.18!$