Given: Pipe P's 1 hour's work = 1/18 Pipe Q's 1 hour's work = 1/12 Calculation: Pipe (P + Q)’s 1 hour's work = 1/18 + 1/12 = 5/36 Pipe (P + Q)’s 7 hours work = 7 × 5/36 = 35/36 ⇒ Remaining work = 1 – 35/36 = 1/36 Then, ⇒ 1/36 × R = 1 ⇒ R = 36 Pipe R's 1 hour's work = 1/36 ⇒ Pipe (P + Q + R)’s 1 hour's work = 1/18 + 1/12 + 1/36 = 1/6 ∴ It will take 6 hours to fill the tank when all three pipes are opened simultaneously.