12. A party of five explorers is on one bank of a stream and a party of five warriors is on the other, and each group wishes to cross to the opposite side. The only means of transportation is a dugout canoe which can hold only three persons. Only one warrior and one explorer know how to paddle. Moreover, for obvious reasons, it is unhealthy for warriors to outnumber explorers in the boat or on a shore. How is it possible for the groups to cross safely? Can it be done if there are six of each?