An iron rod of length 50 cm is joined at an end to aluminium rod of length 100 cm. All measurements refer to 20_0 C . Find the length of the composite system at 100`0` C. and its average coefficent of linear expansion. The coefficient o f linear expansion o f iron and aluminium are 10 xx 10_-6 0 C_-1 and 24 xx10_-6 0 C _-1 respectively.