Answer
$$C_1=60 \ \mu F.$$
Work Step by Step
First, we have
$$\frac{1}{C}=\frac{1}{C_1}+\frac{1}{C_2}\Longrightarrow \frac{1}{C_1}=\frac{1}{C}-\frac{1}{C_2}.$$ Now, by substituting, we have $$\frac{1}{C_1}=\frac{1}{20}-\frac{1}{30}=\frac{3}{60}-\frac{2}{60}=\frac{1}{60}$$ Hence, we get $$C_1=60 \ \mu F.$$
