Answer :
This is a very simple problem. If they rode from the campgrounds to the lake and back that is 2* the distance or distance + distance. The distance was 19 so the total distance is 38km. Now you have to find their average speed in km per hour. So they took 2 hours to go 38km. All you do is divide 38km/2hr. and you get 19 km per hour. :)