4. Three people want to travel from city A to city B, which is 45 kilometers away from A. They have two bicycles. The speed of a cyclist is 15 km/h, and the speed of a pedestrian is 5 km/h. What is the minimum time they can reach B, if the bicycle can be left unattended on the road?
Solution. Two people ride a bicycle for 10 kilometers, then one of them leaves the bicycle by the road and walks the next 10 kilometers, while the other continues for the next 10 kilometers and also leaves the bicycle (which the first one should pick up later), the third one walks the first 10 kilometers and then rides the bicycle picked up by the first one for the remaining distance.

In any case, they will collectively ride 60 km and walk 30 km. Therefore, there will be at least one of them who will walk no less than 10 kilometers and ride the rest. If he walks exactly 10 kilometers and rides 20 kilometers, it will take 3 hours and 20 minutes, and any increase in the 10 kilometers will only worsen the result.

Criteria. A complete solution consists of two parts: a description of how the cyclists should ride, and a proof that this result cannot be improved. If only one of these parts is present, the problem is not solved, and we give 2 points.