According to the formula, if pipe A can comletely fill a tank in x hours and pipe B can empty the same tank in y hours, then the time taken to fill the tank when both the pipes are opened simultaneously is $\frac{\mathrm{xy}}{\mathrm{x}+\mathrm{y}}$hours.

so, ${{}_{}}_{}\frac{20\times 30}{20+30}=\frac{600}{50}=12 {\min }_{}$

The tank will be filled in 12 min when both the pipes are opened simultaneously.

Hence option B is correct