Two pipes X and Y together can fill a tank in 72 minutes. If the size of the pipe X is thrice as Y then Y alone can fill the tank in -

Let,
The time taken by Y alone to fill the tank be A minutes.
Given that, the size of the pipe X is thrice as Y.
Then, X fills the tank in A/3 minutes.
Part filled by X in 1 minute = 1/(A/3)
= 3/A
Part filled by Y in 1 minute = 1/A
Since, X and Y together take 72 minutes.
Part filled by (X + Y) in 1 minute = 1/72
i.e.,
(1/A + 3/A) = 1/72
⇒ 4/A = 1/72
A = 288 minutes
= (288/60) hours
= 4(48/60)
= 4(4/5) hours
= 4 hours and (4/5 x 60) minutes
= 4 hours and 48 minutes.
Hence, the pipe Y alone takes 4 hours and 48 minutes to fill the tank.