کل $P={{P}_{1}}+{{P}_{2}}+{{P}_{3}}\Rightarrow 70=40+10+{{P}_{3}}\Rightarrow {{P}_{3}}=20W$  دو مقاومت ${{R}_{2}}$ و ${{R}_{3}}$ موازی هستند، لذا: \[\left\{ _{P=\frac{{{V}^{2}}}{R}}^{{{V}_{2}}={{V}_{3}}} \right.\Rightarrow \frac{{{R}_{3}}}{{{R}_{2}}}=\frac{\frac{V_{3}^{2}}{{{P}_{3}}}}{\frac{V_{2}^{2}}{{{P}_{2}}}}=\frac{{{P}_{2}}}{{{P}_{3}}}=\frac{10}{20}\Rightarrow \frac{{{R}_{3}}}{{{R}_{2}}}=\frac{1}{2}\]