In parallel combination: Potential difference is same across all $$ \begin{aligned} & \text { Energy }=\frac{1}{2}\left(\mathrm{C}_1+\mathrm{C}_2+\mathrm{C}_3\right) \mathrm{V}^2 \\ & =\frac{1}{2}(25+30+45) \times(100)^2 \times 10^{-6}=0.5=\mathrm{E} \end{aligned} $$ In series combination: Charge is same on all. $$ \begin{aligned} & \frac{1}{\mathrm{C}_{\text {equ }}}=\frac{1}{\mathrm{C}_1}+\frac{1}{\mathrm{C}_2}+\frac{1}{\mathrm{C}_3}=\frac{1}{25}+\frac{1}{30}+\frac{1}{45} \\ & \frac{1}{\mathrm{C}_{\text {equ }}}=\frac{(18+15+10)}{450}=\frac{43}{450} \Rightarrow \mathrm{C}_{\text {equ }}=\frac{450}{43} \\ & \text { Energy }=\frac{q^2}{2 \mathrm{C}_1}+\frac{q^2}{2 \mathrm{C}_2}+\frac{q^2}{2 \mathrm{C}_3} \\ & =\frac{\mathrm{Q}^2}{2}\left[\frac{1}{\mathrm{C}_1}+\frac{1}{\mathrm{C}_2}+\frac{1}{\mathrm{C}_3}\right] \\ & \frac{\left(\mathrm{V} \times \mathrm{C}_{\text {equ }}\right)^2}{2} \times \frac{1}{\mathrm{C}_{\text {equ }}}=\frac{\mathrm{V}^2 \mathrm{C}_{\text {equ }}}{2} \\ & \frac{(100)^2}{2} \times \frac{450}{43} \times 10^{-6} \\ & \Rightarrow \frac{4.5}{86}=\frac{9}{x} E=\frac{9}{x} \times 0.5 \Rightarrow x=86 \end{aligned} $$