A unit scale is to be prepared whose length does not change with temperature and remains 20 cm, using a bimetallic strip made of brass and iron each…

A unit scale is to be prepared whose length does not change with temperature and remains $\displaystyle 20$ 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 $\displaystyle 40$ cm and length of iron will be \_\_\_\_ cm. $\displaystyle \left(\alpha_{\text {iron }}=1.2 \times 10^{-5} \mathrm{~K}^{-1}\right.$ and $\displaystyle \left.\alpha_{\text {brass }}=1.8 \times 10^{-5} \mathrm{~K}^{-1}\right)$.