The monkey can travel by water, carrying all coconuts at once. Because it is an oasis there is fresh water he can drink, and go across in a boat. An oasis is usually found in the desert, and they have water and trees. If the monkey were to take all of the coconuts at once, carrying them onto the boat 1000 at a time, he will not have to eat any coconuts. And if he does, he takes 1500 more.