Properties of Solids and Liquids: JEE Main 2026 Physics question

A string $A$ of length 0.314 m and Young's modulus $2\times10^{10}$ N/m$^2$ is connected to another string $B$ of length and Young's modulus both twice of those of $A$. This series combination of strings is then suspended from a rigid support and its free end is fixed to a load of mass 0.8 kg. The net change in length of the combination is ______ mm. (radius of both the strings is 0.2 mm and acceleration due to gravity $=10$ m/s$^2$) (Mass of both strings is to be neglected as compared to the mass of load)