7.14. In a quarry, 120 granite slabs weighing 7 tons each and 80 slabs weighing 9 tons each are prepared. Up to 40 tons can be loaded onto a railway platform. What is the minimum number of platforms required to transport all the slabs?
7.14. It is impossible to load 6 slabs, each weighing 7 tons, onto one platform. Therefore, at least $200 / 5=40$ platforms are needed. Forty platforms are sufficient: onto each platform, 3 slabs weighing 7 tons each and 2 slabs weighing 9 tons each can be loaded.