Three children need to travel a distance of 84 kilometers on two bicycles. On foot, they can cover 5 kilometers, and on a bicycle, they can cover 20 kilometers per hour. How long will it take for all three of them to reach the destination if only one child can ride a bicycle at a time?
The task text does not specify whether a child sitting on a bicycle can pull another (empty) bicycle. In some papers, this inaccuracy in the original task was corrected, and - from a traffic regulation perspective, absolutely correctly - this possibility was excluded. We also provide the solution with this additional condition.

At every point on the route, all three children can pass, either cycling or on foot. It is also true that at every point, there will be one among them who can pass this point on foot, in other words, the sum of the distances walked by the three children is at least 84 kilometers, which is the total length of the route.

Initially, for the selected point, the segment towards the start of the route has more children than bicycles, and as long as only cyclists pass through here - forward or possibly backward - this remains the case. If, however, all three pass this point, then - since a bicycle cannot move on its own - and we have excluded the movement of a child with two bicycles, someone must have walked past this point.

If now the sum of the walking distances is at least as much as the total route length, then there is a child who walks at least one-third of the total route. If this child cycles the rest of the route, does not wait, and does not turn back either on foot or on a bicycle, then they cannot reach the finish earlier than $\left(\frac{1}{3} \cdot 84\right) : 5 + \left(\frac{2}{3} \cdot 84\right) : 20 = 8.4$ hours. Therefore, no one can reach the finish in less than 8.4 hours.

For all three to complete the 84-kilometer route in 8.4 hours, according to the above calculation, it is necessary that no child walks more than one-third of the route. However, the sum of their walking distances is the total route length, so they can only reach the finish in exactly 8.4 hours if each of them walks exactly one-third of the route and cycles the other two-thirds. (And of course, they must continuously move forward.)

This is achievable, for example, if one child cycles the first and second, another the first and third, and the last the second and third third of the route. The first child reaches the finish on foot, the second by walking the middle third of the route and using the first child's left bicycle, and the third by using the second child's left bicycle for the first third of the route.

The answer to the - supplemented - task is therefore: 8.4 hours.