Ship A leaves port sailing north at a speed of 25km/h. A half hour later, ship B leaves the same port sailing east at a speed of 20km/hr. Let t (in hours) be the time ship B has been at sea.

a) find an expression in terms of t giving the distance between the two ships

b) use the expression obtained in part a to find the distance between the two ships 2h after ship A has left port.