According to Froude's Law, the speed at which an aquatic animal can swim is proportional to the square root of its length. (On Growth and Form, Sir D'Arcy Thompson, Cambridge University Press, 1961.) If a 75-foot blue whale can swim at a maximum speed of 20 knots, write a function that relates its speed to its length. How fast would a similar 60-foot-long blue whale be able to swim?