A rod of length `20 cm` is made of metal. It expands by `0.075 cm` when its temperature is raised from `0^(@)C` to `100^(@)C`. Another rod of different metal `B` having the same length expands by `0.045 cm` for the same change in temperature. A third rod of the same length is composed of two parts, one of metal `A` and the oher of metal `B`. This rod expandss by `0.060 cm` for the same change in temperature. The portion made of metal `A` has the length :