According to the above given 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.

According to the question x+y+z=10

So,

 $$\begin{gathered} 10=\frac{30\times 40\times \mathrm{z}}{(30\times 40)+(40\times \mathrm{z})+(\mathrm{z}\times 30)} \\ 10=\frac{1200\mathrm{z}}{1200+30\mathrm{z}+40\mathrm{z}} \\ 10=\frac{1200\mathrm{z}}{1200+70\mathrm{z}} \end{gathered}$$

Upon cross multiplication,

$$\begin{gathered} 10\left(1200+70\mathrm{z}\right)=1200\mathrm{z} \\ 12000+700\mathrm{z}=1200\mathrm{z} \\ 12000=1200\mathrm{z}-700\mathrm{z} \\ 12000=500\mathrm{z} \\ \mathrm{z}=\frac{12000}{500} \\ \mathrm{z}=24 \mathrm{minutes} \end{gathered}$$

Hence, the tank will be emptied in 24 minutes.

Hence option B is correct.