Let us say that elongation in copper \(=x\)

Than elongation in steel \(=2-x\)

We know

\(\frac{F L}{A Y}=\Delta x\)

\(\because F, A, L\) are same only material is different We can say

\(\frac{1}{Y} \propto \Delta x\)

\(\frac{Y_2}{Y_1}=\frac{\Delta x_1}{\Delta x_2}\)   \(\left\{\begin{array}{l}\text { Where } \\ Y_2=Y_{\text {steel }} \\ Y_1=Y_{\text {copper }} \\ \Delta x_1=\text { elongation in copper }=x \\ \Delta x_2=2-x\end{array}\right.\)

Substituting values

\(\frac{20 \times 10^{11}}{12 \times 10^{11}}=\frac{x}{2-x}\)

\(x=1.25 \,cm\)

So \(\Delta x_{\text {copper }}=1.25 \,cm , \Delta x_{\text {sleel }}=0.75 \,cm\)