According to the formula, if pipe A can comletely empties a tank in x hours and pipe B can comletely empty a tank in y hours, then the time taken by both pipes together to empty the tank is $\frac{\mathrm{xy}}{\mathrm{x}+\mathrm{y}}$ hours.

So, $\frac{60\times 30}{60+30}=\frac{1800}{90}=20 \mathrm{minutes}$

The tank will be empties by both the pipe when opened simultaneously in 20 minutes

Hence option A is correct