Description for Correct answer:
The ratio of the diameters of Pipe A and Pipe B is 1 : 2.

Therefore, the ratio of the rate of flow of water through A and B is 1 : 4

And the ratio of the time for Pipe A and Pipe B to fill a given capacity is 4 : 1.

If Pipe A can fill the tank in 40 minutes, then Pipe B will fill it in 10 minutes.

Together, the two pipes can fill \( \Large \frac{1}{40}+\frac{1}{10}=\frac{1+4}{10}=\frac{1}{8} \)th of tank in a minute.

They will take 8 minutes to fill tank.

Hence, to fill \( \Large \frac{3}{4} \)th of the tank, the two pipes will take \( \Large \frac{3}{4} \times 8 \) = 6minutes.