For capacitors in series, the reciprocal of the equivalent capacitance is the sum of the reciprocals of individual capacitances: $\frac{1}{C_{eq}}=\frac{1}{C_1}+\frac{1}{C_2}$ $\frac{1}{C_{eq}}=\frac{1}{C}+\frac{1}{2C}=\frac{2+1}{2C}=\frac{3}{2C}$ $C_{eq}=\frac{2C}{3}$. Note: In series, the total capacitance is always less than the smallest individual capacitance.