Thermodynamics & KTG

A unit scale is to be prepared whose length does not change with temperature and remains $20 \mathrm{~cm}, using a bimetallic strip made of brass and iron each of different length. The length of both components would change in such a way that difference between their lengths remains constant. If length of brass is 40 \mathrm{~cm} and length of iron will be __________ \mathrm{cm}. \left(\alpha_{\text {iron }}=1.2 \times 10^{-5} \mathrm{~K}^{-1}\right. and \left.\alpha_{\text {brass }}=1.8 \times 10^{-5} \mathrm{~K}^{-1}\right)$.