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, $\frac{10\times 20}{10+20}=\frac{200}{30}=6\frac{2}{3} \mathrm{minutes}$

The tank will be filled by both the pipe opened simultaneously in $6\frac{2}{3}$ hours

Hence option D is correct