6. (6 points) It is known that the work efficiency ratio of water pump A to water pump B is 3:4. If the two pumps work together to empty a water pool, it takes 15 hours to complete the task. Now, if pump B works for 9 hours and then is turned off, and pump A is then turned on, it will take $\qquad$ more hours to empty the pool.
【Analysis】According to "workload = work efficiency $\times$ working time". Given the conditions, let the work efficiencies of A and B be $\frac{3}{4}$ and $1$, respectively. The total workload can be calculated as $\left(\frac{3}{4}+1\right) \times 15=26.25$. Subtracting the workload already completed by B from the total workload gives the remaining workload for B. Using the formula, we can then calculate the working time for A.

【Solution】Let the work efficiencies of A and B be $\frac{3}{4}$ and $1$, respectively.
$$
\begin{array}{c}
\left(\frac{3}{4}+1\right) \times 15=26.25 \\
26.25-1 \times 9=17.25 \\
17.25 \div \frac{3}{4}=23 \text { (hours) }
\end{array}
$$

Therefore: It will take another 23 hours to drain the water pool.