Given: Pipe A can fill in 15 hours. Pipe B can fill in 18 hours. Calculation: Tank filled by Pipe A in 1 hour ⇒ 1/15 Tank filled by Pipe B in 1 hour ⇒ 1/18 Tank filled by both Pipes in 1 hour ⇒ (1/15) + (1/18) ⇒ 11/90 Time taken By both pipes to fill the tank ⇒ 90/11 ⇒ 8