Two exactly similar wires of steel (Y = 20 × 10¹¹ dy/cm²) and copper (Y = 12 × 10¹¹dy/cm²) are stretched by equal forces. If the total elongation is 1cm ,then elongation of copper wire is

Y_st = 20 x 10¹¹ dyne /cm²
Y_cu = 12 x 10¹¹ dyne /cm²
e_st + e_cu = 1cm  ............ (1)
$\mathrm{e}\propto \frac{1}{Y} \frac{{\mathrm{e}}_{\mathrm{st}}}{{\mathrm{e}}_{\mathrm{cu}}}=\frac{Y\mathrm{cu}}{Y_{\mathrm{st}}}=\frac{12 \mathrm{x} {10}^{11}}{20 \mathrm{x} {10}^{11}}$
e_st = 3/5 e_cu ------(2)
sub (2) in (1) 
$$\begin{gathered} \frac{3}{5}{\mathrm{e}}_{\mathrm{cu}}+{\mathrm{e}}_{\mathrm{cu}}=1\Rightarrow 8{\mathrm{e}}_{\mathrm{cu}}=5 \\ {\mathrm{e}}_{\mathrm{cu}}=5/8 \mathrm{cm} \end{gathered}$$