A group of 3 explorers travelled through the new Guinea rain forests with 3 Marind-Anim guides. Their inflatable dinghy could on
ly take 2 people at a time and only one of the explorers and one guide could successfully operate it. Once the explorers realised that the Marind-Anim were renowned head hunters they were not willing to ever be outnumbered by them. In spite of these difficulties they worked out a way to negotiate each river. What is the smallest number of crossings needed to get the party safely across a river?
