JOHN WALKS FROM TOWN A TO TOWN B. HE FIRST WALKS ON FLAT LAND, AND...

15. John walks from town A to town B. He first walks on flat land, and then uphill. He then returns to town A along the same route. John’s walking speed on flat land is 4 km/h. He walks uphill at a speed of 3 km/h and he walks downhill at a speed of 6 km/h. If the entire journey took 6 hours, what is the distance from town A to town B?