\(\Rightarrow F =\frac{ YA }{\ell} \Delta \ell\)
\(\left(\frac{ A \Delta \ell}{\ell}\right)_1=\left(\frac{ A \Delta \ell}{\ell}\right)_2\) \(\Rightarrow \frac{\Delta \ell_2}{\Delta \ell_1}=\frac{ A _1}{ A _2} \times \frac{\ell_2}{\ell_1}\)
\(\Rightarrow \frac{\Delta \ell_2}{0.2}=\frac{1}{2.4 \times 2.4} \times \frac{2}{1}\) \(\Rightarrow \Delta \ell_2=6.9 \times 10^{-2}\,mm\)