行程, Circular track, Catch-up problem.
When A catches up with B from behind for the first time, it takes $300 \div(5-4.2)=375$ seconds;
When A catches up with B from behind for the first time, A has run $5 \times 375=1875$ meters; $1875 \div 300=6 \cdots \cdots 75$, when A catches up with B from behind for the first time, A has already completed 6 laps.
Student A and Student B are practicing running on a 300-meter circular track. The two start at the same point and time, running in the same direction. It is known that A's average speed is 5 meters per second, and B's average speed is 4.2 meters per second. When A catches up with B from behind for the first time, how many laps has A completed?

When A catches up with B from behind for the first time, how many laps has A completed?