In how many ways can a teacher arrange 4 students in the front row of a class room with a total of 30 students

The student on the left end of the row can be any one of 30. 
                         For each of those ...
The student in the next seat can be any one of the remaining 29.
                         For each of those ...
The student in the next seat can be any one of the remaining 28.
                         For each of those ...
The student on the right end of the row can be any one of the remaining 27.

The total number of ways to pick 4 students and seat them in the front row is

            (30 x 29 x 28 x 27) = 657,720 ways.

That's enough to do it a different way every school day for about 3,600 years !
(or until they all graduate, whichever comes first)