Ship A leaves port sailing north at a speed of 20 mph. A half hour later, Ship B leaves the same port sailing east at a speed of 35 mph. Let t (in hours) denote the time ship B has been at sea.
a) Find an expression in terms of t giving the distance D between the two ships.
b) Use the expression obtained in part (a) to find the distance between the two ships 4 hours after A has left port.