Mass of the substance = 900 g
Volume of the substance = 1000 cm3
Density of the substance = Mass / Volume
= 900 g / 1000 cm3
= 0.9 g/cm3
As the density of the substance (0.9 g/cm3) is less than the density of the water (1 g/cm3), the substance will float on water.
Mass of water displaced by the substance = Density of water × Volume of the substance
Mass of water displaced by the substance = 1 g/cm3 × 1000 cm3
= 1000 g