1. Energy when capacitors are in parallel

For parallel combination, $C_p = 25+30+45 = 100\,\mu F$

Given voltage, $V=100\,V$

Energy stored, $E=\frac12 C_p V^2$

So, $E=\frac12 (100\,\mu F)(100)^2$

2. Equivalent capacitance in series

For series combination, $\frac{1}{C_s} = \frac{1}{25}+\frac{1}{30}+\frac{1}{45}$

Taking LCM $=450$, $\frac{1}{C_s} = \frac{18+15+10}{450} = \frac{43}{450}$

Hence, $C_s = \frac{450}{43}\,\mu F$

3. Energy in series combination

At the same voltage $V=100\,V$, $E_s = \frac12 C_s V^2$

Thus, $\frac{E_s}{E} = \frac{C_s}{C_p} = \frac{\frac{450}{43}}{100} = \frac{9}{86}$

So, $E_s = \frac{9}{86}E$

Comparing with $E_s = \frac{9}{x}E$ we get, $x=86$

4. Comparison with stored answer

Stored correct answer = $86$

This matches our derived answer.