The tip speed ratio of a turbine is the ratio R = T/W, where T is the speed of the tip of a blade and W is the speed of the wind. (Engineers have found empirically that a turbine with n blades extracts maximum power from the wind when R = 2π/n.) Calculate dR/dt (t in minutes) if W = 45 km/h and W decreases at a rate of 4 km/h per minute, and the tip speed has constant value T = 150 km/h.