The correct answer is - 25 miles.
The journey/postal/communication relay system established in the Mongol Empire had stations set on approximately 25 miles, though there were variations from around 20 to around 30 miles.
This system was set up because the empire was enormous, so the quick travel of information was key for its functioning and proper organization.
The system, known as yam or ortoo, was very simple but very effective. For the means of travel the horse was the animal that was used, and the Mongols estimated the distance at which a single horse can be giving its maximum in speed, so they put up stations at distances where the horse's speed will start to decline because of exhaustion. The rider of the horse was either giving the message to another rider, or he was taking food and water with him, as well as a new, rested horse, and continued to the other station.
