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

So, $\frac{5\times 4}{5-4}=\frac{20}{1}=20 \mathrm{hours}$

Therefore, the tank will be filled in 20 hours when both the pipes are opened simultaneously.

Hence option C is correct.