Mathematics textbook for the 5th grade. According to the conditions of the problem, every day a Pedestrian goes from point "A" to point "B", on the way he meets a Cyclist. Then he returns home, where his faithful wife is waiting for him. The next day everything repeats. But one day the Investigator stops the Pedestrian and reports that the Cyclist has disappeared. The Investigator asks the Pedestrian to go with him to point "B" in order to interrogate witnesses and figure out this complicated story together.