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

So, $\frac{20\times 40}{40-20}=\frac{800}{20}=40 \mathrm{hours}$

The tank will be filled by both the pipe opened simultaneously in 40 hours

Hence option A is correct