A series combination of 10 capacitors, each of value ' C_1 ' is...

For the series combination of 10 capacitors, each of capacitance $C_1$:

Equivalent capacitance, $C_{eq1} = \dfrac{C_1}{10}$

Potential difference, $V_1 = 4V$

Energy stored, $U_1 = \dfrac{1}{2} C_{eq1} V_1^2 = \dfrac{1}{2} \left(\dfrac{C_1}{10}\right) (4V)^2 = \dfrac{16 C_1 V^2}{20} = \dfrac{4}{5} C_1 V^2$

For the parallel combination of 8 capacitors, each of capacitance $C_2$:

Equivalent capacitance, $C_{eq2} = 8 C_2$

Potential difference, $V_2 = V$

Energy stored, $U_2 = \dfrac{1}{2} C_{eq2} V_2^2 = \dfrac{1}{2} (8 C_2) V^2 = 4 C_2 V^2$

Given that the energies in the two combinations are the same ($U_1 = U_2$):

$\dfrac{4}{5} C_1 V^2 = 4 C_2 V^2$

$\dfrac{4}{5} C_1 = 4 C_2$

$C_2 = \dfrac{C_1}{5}$

Answer: $C_2 = C_1/5$