A helium balloon (in the shape of a sphere) has radius 7.25 m .
For the density of air, please use 1.29 kg/m^3, and for Helium, use .179 kg/m^3
How much additional mass (payload) could this balloon lift? You should assume the balloon's skin, plus other parts of the balloon's structure have a total mass of 920 kg . Note however that this number does NOT yet include the mass of the helium filling the balloon, which you will need to account for!