While exploring for oil, it was necessary to carry at least 18 tonnes of supplies and 80 people into a desert region. Two types of vehicle were available: Landmasters and Sandrovers. Each Landmaster could carry 900 kg of supplies and 6 people. Each Sandrover could carry 1350kg of supplies and 5 people. If there were only 12 of each
type in good running order, what is the smallest number of vehicles necessary for the
journey?