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 full tank will be filled in $\frac{\mathrm{xyz}}{\mathrm{yz}+\mathrm{xz}-\mathrm{xy}}$ hours.

So,

$$\begin{gathered} \frac{30\times 10\times 40}{\left(30\times 40\right)+\left(10\times 40\right)-\left(30\times 10\right)} \\ =\frac{12000}{400+1200-300}=\frac{12000}{1300}=9\frac{3}{13} \mathrm{hours} \end{gathered}$$

The tank will be filled in $9\frac{3}{13}$ hours when all the pipes are opened simultaneously.

Hence option A is correct