Explanation:

- Pipe A can fill the tank in 10 hours, so it fills 1/10 of the tank in an hour.

- Pipe B can fill the tank in 20 hours, so it fills 1/20 of the tank in an hour.

- Pipe C can empty the tank in 8 hours, so it drains 1/8 of the tank in an hour.

- When all pipes are opened together:

- The net fill rate per hour = (1/10 + 1/20 - 1/8).

- Simplified: (2/20 + 1/20 - 2.5/20) = 0.5/20 = 1/40 of the tank per hour.

- Therefore, the tank is filled in 40 hours when all pipes work together.

- Option 3: 40 hr.