A differential wheel and axle has a wheel diameter of 300 mm and axle diameters of 100 and 75 mm. The effort required to raise a load of 50 kg is 55 N and the effort required to raise a load of 200 kg is 180 N. Determine (a) the velocity ratio, (b) the law of the machine, (c) the efficiency of the machine when raising a load of 100 kg.