MSA Education BD

 $\begin{aligned} & \mathrm{V}_{\mathrm{O}}=\mathrm{V}_1+\mathrm{V}_2+\mathrm{V}_3+\mathrm{V}_4 \\ & \text { या, } V_o=\frac{1}{4 \pi \varepsilon_0}\left[\frac{\mathrm{Q}_1}{r}+\frac{\mathrm{Q}_2}{r}+\frac{\mathrm{Q}_3}{r}+\frac{\mathrm{Q}_4}{r}\right] \\ & \text { या, } V_o=\frac{1}{4 \pi \varepsilon_0} \frac{1}{r}\left(Q_1+Q_2+Q_3+Q_4\right) \\ & \text { या, } O=9 \times 10^9 \frac{1}{r}\left(7+7+7+Q_4\right) \\ & \therefore \mathrm{Q}_4=-21 \mathrm{C}\end{aligned}$