15. Person A and Person B start from the same point on a 300-meter circular track, running in opposite directions. A runs 2 meters per second, and B runs 4 meters per second. When they meet head-on, A turns around and runs back; when A and B meet again, B turns around and runs back. If this continues, after (
) seconds, they will meet at the starting point for the first time.
【Analysis】Assume that at the start, A runs clockwise and B runs counterclockwise. The first meeting takes $300 \div(4+2)=50$ seconds, at which point A runs 200 meters clockwise and B runs 100 meters counterclockwise; the second meeting takes $300 \div(4-2)=150$ seconds, at which point A runs 600 meters counterclockwise and B runs 300 meters counterclockwise; the third meeting takes $300 \div(4+2)=50$ seconds, at which point A runs 200 meters counterclockwise and B runs 100 meters clockwise; the fourth meeting takes $300 \div(4-2)=150$ seconds, at which point A runs 600 meters clockwise and B runs 300 meters clockwise; the two first meet at the starting point after a total of 400 seconds.