Tex & Mex
Two friends get in a closed circuit 2 km long with the bicycles. The first one rides with 15 km per hour, while the second just with 12 km per hour. They have also two dogs, Tex and Mex. Assuming that they start in the same direction at the same moment, and from that minute the dogs run constantly from one bicycle to the other and back with a medium speed of 18 km per hour, what distance the two dogs covered when the first rider catches back the second one?