Correct Answer - Option 1 : 10 hours

**Given:**

⇒ Pipe x's 1 hour's work = 1/15

⇒ Pipe y's 1 hour's work = 1/12

⇒ Pipe z's 1 hour's work = 1/20

**Calculation:**

⇒ Pipe (x + y + z)'s 1 hour's work = 1/15 + 1/12 - 1/20 = 1/10

**∴ In 10 hours, hose will get completely filled if all three pipes are opened simultaneously.**