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 = 75 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.