Rate of Change of Distance Between Ships

Two ships, A and B, depart from different ports at the same time. The port for Ship B is located 50 miles directly north of the port for Ship A. Ship A travels east at a constant speed of 20 miles per hour, while Ship B travels south at a constant speed of 15 miles per hour. Two hours after departure, at what rate, in miles per hour, is the distance between the ships changing?