【Solution】Solution: 3 km/h $=50$ m/min. Let the speed of the monkey be $x$ m/min, then:
$$
\frac{3 \mathrm{x}}{\mathrm{x}+50} \times 6=\frac{12 \mathrm{x}}{50}
$$

Solving for $x$ gives: $x=25$
$$
12 \times 25=300 \text { (m) }
$$

Answer: The distance between $A$ and $B$ is 300 meters. Therefore, the answer is: 300 meters.
20. (6 points) A group of monkeys needs to transport peaches from location $A$ to location $B$. Every 3 minutes, a monkey starts from $A$ and heads to $B$, and the entire journey takes 12 minutes. A rabbit starts running from $B$ to $A$ at the same time a monkey arrives at $B$. On its way, the rabbit encounters 5 monkeys coming towards it, and continues to $A$, where it meets another monkey just starting from $A$. If the rabbit's running speed is 3 kilometers/hour, then the distance between $A$ and $B$ is $\qquad$