19. On the number line, points $A, B, C, D$ were marked. It is known that the first three points correspond to the numbers 5, 8, and -10. What number corresponds to point $D$, if when the direction of the number line is reversed, the sum of the four numbers corresponding to these points does not change?
19. Let the point $D$ correspond to the number $x$. The sum of the numbers before changing the direction of the line is $5+8-10+x$, and after changing the direction it is $-5-8+10-x$. According to the condition, $5+8-10+x=5-8+10-x$, from which $2x = -6$ and $x = -3$. The problem can be solved more simply without resorting to an equation. Since the sum did not change when the signs were reversed, it is equal to zero. And if we have the terms 8, 5, and -10, then the fourth term is -3, because in this case $8+5-10-3=0$. Therefore, the point $D$ corresponds to the number -3.