CAL
posted by LAURA .
At noon, ship A is 30 nautical miles due west of ship B. Ship A is sailing west at 21 knots and ship B is sailing north at 15 knots. How fast (in knots) is the distance between the ships changing at 4 PM? (Note: 1 knot is a speed of 1 nautical mile per hour.)

D^2 = x^2 + y^2
2 D dD/dt = 2 x dx/dt + 2 y dy/dt