A military column of 1 km length marches with a constant speed of 6 kilometers per hour. A courier from the back of the column is sent to the head of the column by bike to deliver a message. Arrived at the front, he instantly returns. When he returns at the back, the column has travelled a distance of 1 km since the start of his errand. The courier drove with constant speed.

The question: How fast did the courier bike?

The answer: Click here!