There are 9 chairs in a room on which 6 persons are to be seated, out of which one is guest with one specific chair. In how many ways they can sit: 

5 persons are to be seated on 8 chairs   ${\text{ i.e. }}^8{C}_3\times 5!$  or 6720.

  {Since 5 chairs can be select in ${ }^8{\mathrm{C}}_5$ and then 5 persons can be arranged in 5 ! ways}.