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 

mass of the hydrogen=  15×0.09 = 1.35 kg15×0.09 = 1.35 kg

so mass of hydrogen+balloon  =  1.35 + 7.15 =  8.50 kg

mass of hydrogen+balloon+floating equipment  =  1.35 + 7.15 =  8.50 + x  kg

mass of air  displaced = 15×1.3 = 19.5 kg15×1.3 = 19.5 kg

so   8.5+x= 19.5    

      x=  11 kg

total mass of the floating body is 11 kg