I first draw the model for 3/5 of the adults. I then divided it by 2 with the dotted lines.

Before, adults had 10 dotted boxes, and children had 6 dotted boxes.

After 125 adults left, 2/3 of adults are children. In terms of boxes, there are now 9 dotted boxes for adults.

This means that 1 dotted box left the train, which is the same as 125.

Since there are 6 dotted boxes for children, the number if children is 125×6

Hope it helps!

This is another method of doing this qn using ratio method. Hope this helps.