19. Person A and Person B start from points $A$ and $B$ respectively at the same time, heading towards each other. The speed ratio of A to B is 5:3. After they meet, they continue to their respective destinations, with A reaching point $B$ and B reaching point $A$, and then both immediately turn back along the same route. If the second meeting point is 50 kilometers away from the first meeting point, then the distance between $A$ and $B$ is $\qquad$ kilometers.
【Solution】Solution: Since the speed ratio of A and B is $5: 3$; the total distance is: $5+3=8$;
At the first meeting, the two people together traveled the distance between A and B, with A traveling $\frac{5}{8}$ of the total distance. The meeting point is $\frac{5}{8}$ of the distance between A and B from A,
At the second meeting, the two people together traveled three times the distance between A and B, so A traveled $\frac{5}{8} \times 3=\frac{15}{8}$ of the total distance. The meeting point is $2-\frac{5}{8} \times 3=\frac{1}{8}$ of the distance between A and B from A,
Therefore, the distance between A and B is:
$$
\begin{array}{l}
50 \div\left(\frac{5}{8}-\frac{1}{8}\right) \\
=50 \div \frac{1}{2} \\
=100 \text { (km) }
\end{array}
$$

Answer: The distance between A and B is 100 km.
So the answer is: 100.