Answer
$20 \space ohm$
Work Step by Step
Since, the resistances are connected in parallel combination.
Formula to calculate the total resistance, we have
$\frac{1}{R_{total}}=\frac{1}{R_1}+\frac{1}{R_2}$
As we are given that $R_1=30 \space ohm$ and $R_{total}=12 \space ohm $
Thus, $\frac{1}{R_{total}}=\frac{1}{R_1}+\frac{1}{R_2}$
$\frac{1}{12}=\frac{1}{30}+\frac{1}{R_2}$
$\frac{1}{R_2}=\frac{1}{12}-\frac{1}{30}$
$R_2=20 \space ohm$
