A helium baloon is used to lift something which weighs 100N. The layer of the baloon weighs 50N, and the volume of the baloon when it is fully blown up is 32 cubic meters. The temperature of the air surrounding it is 0 degrees celsius. The baloon is filled with so much helium that the net force exacted on it as well as the weight it is lifting is 30N. How many mol helium is there in the gas? Please give me a detailed explanation as to how you obtain your answer.