A road running north to south crosses a road going east to west at the point P. Car A is driving north along the
first road, and car B is driving east along the second road. At a particular time car A is 40 miles to the north
of P and traveling at 60 mi/hr, while car B is 30 miles to the east of P and traveling at 75 mi/hr. How fast is the
distance between the two cars changing?