\(Y=\frac{m g / A}{\Delta \ell / \ell}=\frac{m g \ell}{A \Delta \ell}\)
Also \(\Delta \ell=\ell \alpha \Delta T\)
From (1) and (2)
\(Y=\frac{m g \ell}{A \ell \alpha \Delta T}=\frac{m g}{A \alpha \Delta T}\)
\(\therefore m=\frac{Y A \alpha \Delta T}{g}\)
\(=\frac{10^{11} \times \pi\left(10^{-3}\right)^2 \times 10^{-5} \times 10}{10}=\pi \approx 3\)