[tex]\Delta l_{1}=10 cm=0,1 m\\\Delta l_{2}=20 cm=0,2 m\\ E_{p1} =\frac{k*\Delta l_{1} }{2} = > k=\frac{2E_{p1}}{\Delta l_{1}} \\E_{p2} =\frac{k*\Delta l_{2} }{2}=\frac{2E_{p1}}{\Delta l_{1}}*\frac{\Delta l_{2}}{2}= > E_{p2}=\frac{E_{p1}\Delta l_{2}}{\Delta l_{1}}=\frac{25*0,2}{0,1}=50J= > E_{p2}=50 J[/tex]