(a) $l \propto \frac{1}{Y} \Rightarrow \frac{Y_{s}}{Y_{c}} = \frac{l_{c}}{l_{s}} \Rightarrow \frac{l_{c}}{l_{s}} = \frac{2 \times 10^{11}}{1.2 \times 10^{11}} = \frac{5}{3}$ …(i)
Also $l_{c} - l_{s} = 0.5$ …(ii)
On solving (i) and (ii) $l_{c} = 1.25cm$ and $l_{s} = 0.75\ cm$