A weighing machine is a sort of spring balance which measure the weight and not the mass of a body. When we stand on the weighing machine, our weight which is due to gravitational attraction of the earth acts vertically downwards. But the buoyancy due to air on our body acts vertically upwards. As a result of this, our apparent weight true weight – buoyant force is less than the true weight. Since the weighing machine measures the apparent weight, our true weight is more, i.e., more than 42 kg.