Explanation:

- Pipe A and B together fill the tank in 20 hours.

- In 1 hour, Pipes A and B fill 1/20 of the tank.

- Pipe C can empty 40% of the tank in 20 hours.

- In 1 hour, Pipe C empties 40% of 1/20 = 1/50 of the tank.

- Net work by all pipes in 1 hour: (1/20 - 1/50).

- Simplifying: \( \frac{5 - 2}{100} = \frac{3}{100} \).

- Together, they fill 3% of the tank per hour.

- To fill 60% of the tank: \( \frac{60}{3} = 20 \) hours.

- Correct Answer: Option 1 - 20 hours

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