- 2 hours
- 4 hours
- 6 hours
- 8 hours

Option 2 : 4 hours

**Concept:**

Work = 1/Efficiency

**Calculation:**

⇒ Pipe A can fill the tank in 6 hours, and Pipe A and B together can fill the tank in 12 hours.

⇒ Pipe B can empty the tank = 1/(1/12 – 1/6) = -12 hours

⇒ In 8 hours A will fill tank = 8/6 = 4/3

⇒ A will fill the extra water = 1 – 4/3 = -1/3

⇒ This extra water will be removed by pipe B in = (-1/12)/(-1/3) = 1/4

⇒ B will be closed after 4 hours

**∴ The required result will be 4 hours.**

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