HI SUrya Kant. Sorry for delayed reply. Let us see this problem. As they are 9 persons and they leave in group of 2 , 3 and 4 onle, They will leave lift on three floors only,neither less nor more (here i mean that a group of 2 and another group of 3  will not leave on the same floor as that means 5 persons are leaving on the same floor.)

Number of ways to choose these three floors is ¹⁰C₃ = 120. Once these three floors are chosen number of ways in which they can leave (ie. 234 or 243 or 324 etc) is 3!=6

 

So total number of ways is 120*6=720