a) What is the minimal mass of helium (density 0.18 kg/m^3) needed to lift a balloon carrying two people in a basket, if the total mass of people, basket, and balloon (but not gas) is 330kg? The density of the air is 1.2 kg/m^3.
b) Repeat for a hot-air balloon whose air density is 10% less than that of the surrounding atmosphere.