A piece of brass (Cu and Zn) weighs 12.9 g in air. When completely immersed in water, it weighs 11.3g. Then relative densities of Cu and Zn are 8.9 and 7.1 respectively. The mass of copper in the alloy is

Loss of weight (12.9 - 11.3) gf = 1.6 gfWeight of water dispalced = 1.6 gfIf m is the mass of Cu, then[m8.9+12.9-m7.1]g = 1.6 gor 7.1 m + 12.9 x8.9 -8.9m = 1.6 x 8.9 x7.1 or 1.8m = 114.8 - 101.1or m = 7.6 gram