A man who lives in a desert sends his servant out to collect the flag which is 4 miles away from the man’s home. To survive the servant must drink 1 litre of water on every mile he walks( so the amount of water is proportional to the distance walked). To do so he can leave caches of water at any time he wants. If we assume theres is an infinite amount of water at the man’s home how can the servant walk 4 miles to the flag, collect it and get back without dying and drinking as few water as possible? Wanted is the smallest amount of water in litres that is needed to do this.

So the “alleged” solution is 11.5 litres but I dont understand how to only use this amount because me and my friends showed that the miminum is 12 so I’m confused. It would be really nice if somebody could help me solve this.