REVEAL THE ANSWER
The explorer can cross the desert using only two porters. They all leave one morning, each with 4 days food. At the end of the first day, each has 3 days food left. The first porter goes back with one days food, leaving 4 days food each for the explorer and the second porter. At the end of the second day, the two men each have 3 days food. The second porter goes back with 2 days food. The explorer goes on with 4 days food, enough to reach his goal.