Two exactly similar wires of steel $\left(\mathrm{y}=20\times {10}^{11}\mathrm{dy}/{\mathrm{cm}}^2\right)$and copper $\left(\mathrm{y}=12\times {10}^{11}\mathrm{dy}/{\mathrm{cm}}^2\right)$are joined together and stretched by a constant force. If the total elongation is 1cm, then the elongation of copper wire is