Thermodynamics Part 3

A unit scale is to be prepared whose length does not change with temperature and remains 20cm, 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 40cm and length of iron will be cm.(α‌iron ‌=1.2×10−5K−1 and α‌brass ‌=1.8×10−5K−1)