Given: Pipe X can fill a tank in 12 hours and pipe Y can fill the same tank in 36 hours Formula used: Time taken to fill = total work/one hour’s work Calculation: Pipe X → 12 hours Pipe Y → 36 hours On taking LCM (12, 36), Total work = 36 One hour’s work of Pipe X = 36/12 = 3 One hour’s work of Pipe Y = 36/36 = 1 One hour’s work of (X + Y) = 3 + 1 = 4 ∴ Time taken by (X + Y) together fill full tank = 36/4 = 9 hours ⇒ Time taken by (X + Y) together fill one-third part of the tank = 9/3 = 3 hours