$Y$ is young modulus of elasticity then change in length in $dx$ element is $dy$ $\mathrm{Y} \times$ strain $=$ stress

$\mathrm{Y} \times \frac{\mathrm{dy}}{\mathrm{dx}}=\frac{\mathrm{T}}{\mathrm{A}}$

$\mathrm{Y} \times \frac{\mathrm{dy}}{\mathrm{dx}}=\frac{\mathrm{mgx}}{\ell \times \mathrm{A}}$

$\int_{0}^{y} Y d y=\int_{30}^{70} \frac{m g}{\ell A} \times x d x$

$\mathrm{Y} \mathrm{y}=\frac{\mathrm{mg}}{\ell \mathrm{A}}\left[\frac{(70)^{2}-(30)^{2}}{2}\right]$

$\mathrm{y}=\frac{\mathrm{mg}}{\mathrm{AY} \times 100} \times 2000$

$\mathrm{y}=\frac{\mathrm{mg}}{\mathrm{AY}} \times 20$

Total length is $=40 \mathrm{cm}=20 \frac{\mathrm{mg}}{\mathrm{AY}} \mathrm{cm}$