According to the above given 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 $\frac{\mathrm{xy}}{\mathrm{y}+\mathrm{x}}$ hours.

so, $\frac{20\times 30}{20+30}=\frac{600}{50}=12 \mathrm{minutes}$

Therefore, the tank will be filled in 12 minutes when both the pipes are opened simultaneously.

Hence option B is correct.