Answer:
Let z = 200 ft and y = 150 ft
Also since 60 mph = 88 ft/sec then 4 mph = 88/15 = 5.87 ft.sec
Now x^2 + y^2 = z^2
x dx/dt + y dy/dt = z dz/dt differentiate wrt t
Or x dx/dt = z dz/dt since dy/dt = zero
x = (200^2 - 150^2)^1/2 = 132.3 ft
dz/dt = x / z * dx/dt = 132.3 / 200 * 5.87 = 3.88 ft/sec
