15. Person A and Person B start from the same point on a 300-meter circular track, running in opposite directions. Person A runs 2 meters per second, and Person B runs 4 meters per second. When they meet head-on, Person A turns around and runs back; when A and B meet again, Person B turns around and runs back. If this continues, they will meet at the starting point after ( ) seconds.
【Answer】 250
【Analysis】Multiple encounters on a circular track require graphical analysis.
As shown in the figure, points A, B, and C divide the circular track into three equal parts. Two people start from point A, and after walking together for one lap, they meet for the first time at point B; after the first meeting, person A turns around, and person B chases person A for one lap, meeting for the second time at point B again; after the second meeting, person B turns around, and the two walk together for one lap, meeting at point A, the starting point.
The first time: $300 \div(2+4)=50$ (seconds), the second time: $300 \div(4-2)=150$ (seconds), the third time: 50 seconds, totaling 250 seconds;