Trig application question -- Help

From a position 110km northwest of a coast guard station, an oil tanker makes radio

contact with the coast guard. The tanker is traveling due south at 25km/h. The radar

unit at the coast guard station has a range of 90km. For what length of time can the

coast guard expect the tanker to be visible on the radar screen, to the nearest tenth

of an hour?

Answer(Suppose to be, anyway): 3.6h

Can someone please explain how to do this problem correctly? i worked it out before but i keep getting 4.5h as the answer. I cant think of any alternate ways of tackling it and I keep getting 4.5h, how do you solve this problem?

any help greatly appricated,

~Coukapecker~