According to the above given formula, if pipe A can comletely fill a tank in x hours and pipe B can comletely fill a 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, $\frac{4\times 5}{5-4}=\frac{20}{1}=20 \mathrm{minutes}$

Therefore, the tank will be filled by both the pipe when opened simultaneously in 20 minutes.

Hence option C is correct.