According to the formula, if pipe A can comletely fils 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, $\frac{36\times 45}{36+45}=\frac{1620}{81}=20 \mathrm{hours}$

The tank will be filled by both the pipe opened simultaneously in 20 hours

Hence option C is correct