how many ways can 3 students sit in a row of 21 chairs

Question

how many ways can 3 students sit in a row of 21 chairs for a photograph

Answer 1

21! / 3! 18! =

 

21*20*19/3*2 =

 

 7*10*19 =

 

70*19

 

70*(20 - 1) =

 

70*20 - 70*1 =

 

1400 - 70

 

1330