According to the formula, If pipe M  and N fills a tank in x and y hours respectively, but a third pipe O 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,

 $$\begin{gathered} \frac{40\times 20\times 10}{(40\times 10)+(20\times 10)-(40\times 20)} \\ =\frac{8000}{400+200-800}=\frac{8000}{200}=40 \mathrm{hours} \end{gathered}$$

Hence, the tank will be filled in 40 hours when all the pipes are opened simultaneously.

Hence option B is correct.