In the days before the invention of aeroplanes European explorers faced grave difficulties exploring the foodless and nearly impenetrable jungles of New Guinea.

With time and experience, they learnt that a well trained and physically fit explorer could carry a three days supply of food with him.

Show how a team of 3^N-1 explorers could send one of their number a distance N days march from the base camp, with all explorers returning safely to the base camp. (Note that once an explorer returns to camp, he must stay there.)