6According 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 $\frac{\mathrm{xyz}}{\mathrm{yz}+\mathrm{xz}-\mathrm{xy}}$ hours.

So,

 $$\begin{gathered} \frac{30\times 40\times 50}{(40\times 50)+(30\times 50)-(30\times 40)} \\ =\frac{60,000}{2000+1500-1200}=\frac{60,000}{2300} \mathrm{or} 26\frac{2}{3}\mathrm{hours} \end{gathered}$$

Hence, the tank will be filled in $26\frac{2}{3}$ hours when all the pipes are opened simultaneously.

Hence option B is correct.