The seventh-grade students of a school traveled to the suburbs on foot.

S= 12*2=24 (km)

T=(4-2)/(6-4)= 1 (hour)

First of all, when the team behind set out, the team in front had already gone 4× 1=4 km, and the speed difference between the two teams was 2, which was easy to find.

The second question, regardless of how many times the cyclist walked back and forth, simply think about it. His speed is 12, and he walked for two hours. It is easy to calculate that the distance he walked is 12×2=24.

The third problem is that when the two teams meet 2 kilometers, they meet 4 kilometers at first, that is, the distance is 2 kilometers, so the time is 2/(6-4)= 1 hour, that is, they leave 1 hour, and the two teams are 2 kilometers apart.