The resultant capacitance of parallel combination of two capacitors \( C_1 \) and \( C_2 \) is 20 µF. When these capacitors are individually connected to a voltage source of 1V, then the energy stored in \( C_2 \) is 9 times that of \( C_1 \). If these two capacitors are connected in series, the resultant capacitance value is: