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, $\frac{10\times 6}{10-6}=\frac{60}{4}=15 \mathrm{hours}$

The tank will be emptied in 15 hours when both the pipes are opened simultaneously.

Hence option A is correct