A `1m` long rod having uniform area of cross section is made of four materials. The first `0.2 m` is made of iron, the next `0.3m` of lead, the following `0.2m` of aluminium and the remaining part is made of copper. Find the centre of mass of the rod. The densities of iron, lead, aluminium and copper are `7.9xx10^3 kg//m^3,11.4xx10^3 kg//m^3, 2.7 xx 10^3 kg//m^3 "and" 8.9 xx 10^3kg//^3` respectively.