A wind turbine rotates at 15 rpm under steady winds; air flows through it at 42 000 kg/s. The measured blade-tip velocity is 250 km/h and the turbine produces 180 kW. Taking the air density as 1.31 kg/m^{3}, determine (a) the average velocity of the air (in m/s) and (b) the conversion efficiency of the turbine (in %).