Correct Answer - Option 3 : 13

\(\frac{1}{2}\) minutes

**Given:**

Pipes A and B can fill a tank in 18 minutes and 27 minutes, respectively.

When A, B and C are opened together, the empty tank is completely filled in 54 minutes.

**Calculation:**

Pipe A's 1 minute's work = 1/18

Pipe B's 1 minute's work = 1/27

Let pipe C's 1 minute's work be 1/C.

⇒ (1/18 + 1/27) - 1/C= 1/54

⇒ 1/C = 2/27

**∴ Pipe C can alone empty the tank in 27/2 = 13\(\frac{1}{2}\) minutes.**

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When a pipe is emptying the tank then it is considered as negative work.