Answer:
Mass = volume * density
tables show air = 1.293 g / L and He = .178 g / L
Each balloon would have a lifting power (using mass) of
(1.293 - .178) = 1.115 g / L = 1.115 kg / m^3
Note there are 1000 g in a kg and 1000 L in a m^3
V (balloon) = 4/3 pi .15^3 = .0141 m^3
Each balloon can lift .0141 m^3 * 1.115 kg/m^3 = .0158 kg
Or 63.4 balloons can lift 1 kg
To lift 75 kg requires 63.4 * 75 = 4760 balloons
