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 $\frac{\mathrm{xy}}{\mathrm{y}-\mathrm{x}}$ hours.

so, $\frac{12\times 20}{20-12}=\frac{280}{8}=30 \min$

Hence, full tank will be filled in 30 min when both the pipes are opened simultaneously.

But the question is to calculate the time taken to fill half tank.

So, $\frac{30}{2}=15 \min$

Hence option B is correct.