Explanation:

- A can fill the tank in 30 hours, so in 1 hour, A fills 1/30 of the tank.

- B can fill the tank in 40 hours, so in 1 hour, B fills 1/40 of the tank.

- C can drain the tank in 60 hours, so in 1 hour, C drains 1/60 of the tank.

- Combined, A, B, and C fill (1/30 + 1/40 - 1/60) of the tank per hour.

- Simplifying, (4/120 + 3/120 - 2/120) = 5/120 = 1/24 of the tank per hour.

- In 20 hours, they fill 20 * 1/24 = 20/24 = 5/6 of the tank.

- After 20 hours, A stops, leaving B and C working.

- B and C fill (1/40 - 1/60) = 1/120 of the tank per hour.

- To fill the remaining 1/6 of the tank, (1/6 * 120) = 20 hours is required.

- Total time = 20 + 20 = 40 hours.

Correct Answer: Option 3 - 40 hours