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 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 cm and length of iron will be $\_\_\_\_$ cm $$ \left(\alpha_{\text {iron }}=1.2 \times 10^{-5} \mathrm{~K}^{-1} \text { and } \alpha_{\text {brass }}=1.8 \times 10^{-5} \mathrm{~K}^{-1}\right) . $$