According to the formula, If pipe A  and B fills a tank in x and y hours respectively, but a third pipe C empties the full tank in z hours then the net portion filled in 1 hour when all of them are opened together is $\frac{\mathrm{xyz}}{\mathrm{xy}+\mathrm{yz}+\mathrm{zx}}$hours.

So,

 $$\begin{gathered} \frac{8\times 6\times 20}{(8\times 6)+(8\times 20)+(6\times 20)} \\ =\frac{960}{328}=\frac{120}{41} \mathrm{or} 2\frac{38}{41}\mathrm{hours} \end{gathered}$$

The tank will be filled in $2\frac{38}{41}$ hours when all the pipes are opened simultaneously.

Hence option C is correct