4. (3 points) A swimming pool has three water inlet pipes, A, B, and C. If only pipe A is opened, it takes 20 hours to fill the pool; if pipes A and B are opened together, it takes 8 hours to fill the pool; if pipes B and C are opened together, it takes 6 hours to fill the pool. Then, if only pipe C is opened, it will take $\qquad$ hours to fill the pool.
4. (3 points) A swimming pool has three water inlet pipes: A, B, and C. If pipe A is opened alone, it takes 20 hours to fill the pool; pipes A and B together take 8 hours to fill the pool; pipes B and C together take 6 hours to fill the pool. Then, if pipe C is opened alone, it takes _ $10 \frac{10}{11}$ _ hours to fill the pool.
【Solution】Solution: The efficiency of pipe B: $\frac{1}{8}-\frac{1}{20}=\frac{3}{40}$,
The efficiency of pipe C: $\frac{1}{6}-\frac{3}{40}=\frac{11}{120}$,
The time for pipe C: $1 \div \frac{11}{120}=10 \frac{10}{11}$ (hours);
Answer: If pipe C is opened alone, it takes $10 \frac{10}{11}$ hours to fill the pool.
Therefore, the answer is: $10 \frac{10}{11}$