1. The problem statement, all variables and given/known data How many helium-filled balloons would it take to lift a person? Assume the person has a mass of 76 kg and that each helium-filled balloon is spherical with a diameter of 33 cm. 2. Relevant equations P = P0 + ρgV 3. The attempt at a solution I really don't know why this is confusing me, since it seems pretty straight forward. But, I know that the buoyant force has to be equal to at least the mass of the person (M) + the mass of the helium in all the balloons (m) which means that FB = Mg + mg and the buoyant force is due to the helium, so, in order for the person to be lifted, ρHegV = Mg + mg so ρHeV = M + m which is ρHeVHe = M + ρairVHe right? So then M/(ρHe - ρair) = V but this would be negative....What I am doing wrong? (I don't need help with the last part which is finding the number of balloons) thanks!