Correct Answer - Option 3 : 36 hours

**Given:**

Pipe A and Pipe B together can fill a tank in = 12 hours

Pipe B alone can fill the tank in = 18 hours

**Calculation:**

Pipe (A + B)’s 1 day’s work = 1/12

Pipe B’s 1 day’s work = 1/18

∴ Pipe A‘s 1 day’s work = 1/12 - 1/18

⇒ (3 – 2)/36

⇒ 1/36

⇒ 36 hours

**∴**** Pipe A alone takes 36 hours to fill the tank. **