Explanation:

- Given, pipes A and B together fill the tank in 3 hours. This means their combined rate is 1/3 of the tank per hour.

- All three pipes, A, B, and C, fill the tank in 4 hours. Thus, their combined rate is 1/4 of the tank per hour.

- Let the rate at which Pipe C empties the tank be 1/x of the tank per hour.

- The combined rate of A and B is 1/3, and the rate of A, B, and C is 1/4, so we have:

$$

\frac{1}{3} - \frac{1}{x} = \frac{1}{4}

$$

- Solving for x:

$$

\frac{1}{x} = \frac{1}{3} - \frac{1}{4} = \frac{4-3}{12} = \frac{1}{12}

$$

- Therefore, x = 12. Pipe C can empty the tank in 12 hours.