A military column 1 kilometer long marches at a constant speed of 6 kilometers per hour. A courier is sent from the back of the column to the front by bike to deliver a message. Upon arriving at the front, he instantly returns. When he gets back to the rear, the column has traveled a distance of 1 kilometer since the start of his errand. The courier cycled at a constant speed.

The question: How fast did the courier bike?

The solution: Click here!