Answer to a) part :
Number of choices to select a person for 1st seat = C(20,1) = 20 ways [ As number of seats(choices) available to 1st person = 20 seats]
Number of choices to select a person for 2nd seat = C(19,1) = 19 ways
and so on till allocation of last seat
Hence number of ways = 20.19.18.........1
= 20 !
B) part is doubtful to me hence did not solve it..