Some potatoes are being transported from the farm by a truck at a speed of 65 kph while the rest are being transported by an ox-cart at a speed at 5 kph. After a while, a horse-cart with speed 13 kph takes the potatoes off the truck, allowing the truck to go back and take the potatoes from the ox-cart. The transfer of potatoes from the ox-cart into the truck takes no time. The horse-cart and the truck arrive simultaneously in the city, which is 100 km from the farm. For how many hours have the potatoes been on the road?