Emptying cylindrical tanks with identical outlet pipes

A cylindrical tank of radius \(1.2\) m holds water to a depth of \(2.5\) m. With \(3\) identical outlet pipes open, the tank empties in \(40\) minutes. Assume that every pipe always drains water at the same constant rate.

1. Find the time taken to empty the same tank using \(5\) pipes.[2]
2. A second cylindrical tank of radius \(0.9\) m holds water to a depth of \(3.2\) m. Find the least number of these pipes that must be opened to empty it in at most \(20\) minutes.[4]
3. In practice, water drains faster when the water is deeper, and the second tank is filled to a greater depth than the first. Explain whether your answer to (b) is still enough, and whether one pipe fewer might also be enough.[2]