I' = \frac{V_1'}{20},\qquad\frac{V_1'}{20} + \frac{V_1' - V_2'}{30} = 4,\qquad\frac{V_2'}{10} + \frac{V_2' - V_1'}{30} = 0.5I'
\displaystyle V_1' = \frac{640}{11}(\mathrm{V}),\quad V_2' = \frac{280}{11}(\mathrm{V})
\displaystyle\frac{V_2'' - 60}{50} + \frac{V_2''}{10} = 0.5I'',\qquad I'' = \frac{V_2'' - 60}{50}
\displaystyle V_2'' = \frac{60}{11}(\mathrm{V})
\displaystyle V_0 = V_2' + V_2'' = \frac{340}{11}(\mathrm{V})

