about variations. 20points thx

for the first hour, the parking fee $C of a van in car park X is $A. thereafter, the fee $B varies directly as the parking time on a half-hourly basis. when a van is parked in car park X for 3 hours, the parking fee is $68; when a van is parked for 4 hours, the parking fee is $92. assume that a van has been parked in car park X for n hours where n ≥ 1.

a) (i) express B in terms of n.
(ii) express C in terms of n.

b) if a van has been parked in car park X for 10 hours, how much is the parking fee?

c) for car park Y, the parking fee $F of a van varies directly as the parking time on a half-hours basis. when a van is parked in car park Y for 6 hours, the parking fee is $120. After how long should the van be parked at least such that the parking fee charged by car park X will be higher than that charged by car park Y?