Thermometry & Thermal Expansion question from JEE Main 2022 (25 Jul, Shift 1)

A unit scale is to be prepared whose length does not change with temperature and remains $20\text{ cm}$, 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 $40\text{ cm}$, then length of iron will be __________ $\text{cm} \cdot \left(\alpha\right.$iron $= 1.2 \times 10^{-5}\text{ K}^{-1}$ and $\alpha$brass $= 1.8 \times 10^{-5}\text{ K}^{-1}\Big)$.