Explanation:

- Pipe A can fill the tank in 18 minutes, so it fills 1/18 of the tank per minute.

- Pipe B can fill the tank in 24 minutes, so it fills 1/24 of the tank per minute.

- Together, they fill \( \left(\frac{1}{18} + \frac{1}{24}\right) \) of the tank per minute.

- This simplifies to \(\frac{4 + 3}{72} = \frac{7}{72}\).

- They need to fill the tank in 12 minutes.

- In 12 minutes, both pipes together fill \( \left(\frac{7}{72} \times 12\right) = \frac{7}{6}\) of the tank if kept open.

- But for full tank, they should fill 1 tank =

=>\( \frac{7}{72} * b + \frac{12-b}{18} = 1\)

\(b=8\)

- Option:3 - 8 minutes is the time after which pipe B should be closed.

Correct Answer: Option 3 - 8 minutes