How fast did the courier bike, when he delivers a letter on moving column? 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 traveled a distance of 1 km since the start of his errand. The courier drove with constant speed.