Question

How to find the time taken to fill of the lower container?

Answer 1

Here you have the solution. If you are interested in the law which I have used here, you can read about countinuity of fluid principle.

Answer 2

Volume of lower cylinder = π * r^2 * 2h = 2πr^2h
Volume of upper cylinder = π * (2r)^2 * h = 4πr^2h
Volume of container = 2πr^2h + 4πr^2h = 6πr^2h
It took 18 minutes to fill the container of volume 6πr^2h, the it will take 2 * 18 / 6 = 6 minutes to fill the lower container of 2πr^2h.
Answer: 6 minutes