Answer
$C = 7.5~\mu F$
Work Step by Step
We can find the equivalent capacitance $C_p$ of the two capacitors in parallel:
$C_p = 20~\mu F + 10~\mu F$
$C_p = 30~\mu F$
We can find the equivalent capacitance $C$:
$\frac{1}{C} = \frac{1}{10\times 10^{-6}~F}+\frac{1}{30\times 10^{-6}~F}$
$\frac{1}{C} = \frac{3}{30\times 10^{-6}~F}+\frac{1}{30\times 10^{-6}~F}$
$\frac{1}{C} = \frac{4}{30\times 10^{-6}~F}$
$C = \frac{30\times 10^{-6}~F}{4}$
$C = 7.5~\mu F$
