Question

A weather forecasting plastic balloon of volume 15 m3 contains hydrogen of density0.09kg m3 . The volume of equipment carried by the balloon is negligible compared to its own volume . The mass of the empty balloon is 7.15kg the balloonis floating

Answer 1

The total mass of the balloon with the Hydrogen gas in it 
  = 15 m^3 * 0.09 kg/m^3 + 7.15 kg
  = 8.50 kg

As the balloon is floating :  air density >  8.50 kg/15 m^2 = 0.507 kg/m^3