Part of the tank filled by tap in 1 hour is $\frac{1}{12}$

Part of the tank empties by leakage in 1 hour is $\frac{1}{20}$

Net part filled in 1 hour when both tap and leakage work together is

$\frac{1}{\mathrm{x}}=\frac{1}{12}-\frac{1}{20}=\frac{5-3}{60}=\frac{2}{60}=\frac{1}{30}$

 x=30 hours

 Hence option C is correct