At proportional limit
Stress \(\propto\) strain
Stress \(=Y \times\) strain \(\{Y=\) Young's Modulus \(\}\)
Stress \(=Y \times \frac{\Delta L}{L}\) \(\left\{\begin{array}{l}\text { Stress }=8 \times 10^8 \,N / m ^2 \\ Y=2 \times 10^{11} \,N / m ^2 \\ L=1 \,m \end{array}\right.\)
Substituting values
\(\frac{8 \times 10^8 \times 1}{2 \times 10^{11}}=\Delta L\)
\(4 \,mm=\Delta L\)