3. (10 points) A water pool has two taps, $A$ and $B$. If $A$ and $B$ are both turned on, it takes 30 minutes to fill the pool. Now, if $A$ and $B$ are both turned on for 10 minutes, and then $A$ is closed, $B$ continues to fill the pool for 40 minutes, the pool can also be filled. If only tap $B$ is turned on to fill the pool, it will take $\qquad$ minutes to fill the pool.
3. (10 points) A water pool has two water taps, $A$ and $B$. If $A$ and $B$ are both turned on, it takes 30 minutes to fill the pool. Now, if $A$ and $B$ are both turned on for 10 minutes, and then $A$ is closed, $B$ continues to fill the pool for 40 minutes, the pool can also be filled. If only $B$ tap is turned on to fill the pool, it takes 60 minutes to fill the pool.
【Analysis】We consider the capacity of the water pool as unit "1". First, we need to find the work efficiency of tap $B$, and then divide unit "1" by the work efficiency of $B$ to get the time required to fill the pool with only $B$ tap open.
【Solution】Solution: $1 \div\left[\left(1-\frac{1}{30} \times 10\right) \div 40\right]$,
$$
\begin{array}{l}
=1 \div \frac{1}{60}, \\
=60 \text { (minutes); }
\end{array}
$$

Answer: If only $B$ tap is turned on to fill the pool, it takes 60 minutes to fill the pool. Therefore, the answer is: 60.