There is a row of soldiers that is 1km in length and they walk with a constant speed in a straight line, in one direction.

All the way at the end walks a messenger. He has to bring a message to the captain walking all the way at the beginning of the row.

The messenger starts walking past the soldiers and immediately turns around when arriving at the captain and walks back to the end of the row. When the messenger is back at the end, the whole group of soldiers have traveled a distance of 1 km.

The soldiers and captain are walking at the same constant speed. The messenger (walking faster then the soldiers) is also walking a a constant speed.

You don’t know anything about time or speed. How far did the messenger travel from the end of the row to the beginning and back?

The messenger has to have traveled 2 km.

It doesn’t matter what speed they walked at. At the beginning of the puzzle, the line is 1 km long. The general is therefore 1 km ahead of him.

The messenger must therefore travel more than 1 km to reach the general. Since the line moves 1 km forward, the end is where the beginning was. Even if he walked 1.5 km to the general, he only has to walk 0.5 km to get back to the end of the line. It goes faster going back, because now they are coming towards him, and not going away.