The densities of freshwater and saltwater will be designated by ρf=1000kg/m3 and ρs=1025kg/m3 respectively. Volume V of the drum=πr2h where r=radius=0.3m, h=height (length)=1m. V=0.09π m3. If M=the mass of the contents, the total mass of the filled drum=20+M kg. This is also the mass of water displaced. Volume of the water displaced depends on the density ρ of the water=mass/volume, so the volume v of water=mass/ρ m3=(20+m)/ρ. This volume has to equal V/2 so that the drum floats with half its volume submerged. Therefore V/2=(20+M)/ρ.
M=ρV/2-20=0.045πρ-20=0.1414ρ-20 kg approx.
(a) ρ=ρs=1025kg/m3: M=124.91 kg.
(b) ρ=ρf=1000kg/m3: M=121.37 kg.