【牛顿问题】 有甲、乙两块草地,两块草地上的草长得一样地密、一样地快。已知30头牛恰好2个月吃完甲草地上的草;若改放20头牛,那么甲草地恰好够吃4个月。如果乙草地面积是甲草地的三倍,那么要使乙草地的草恰好够吃半年,试问在乙草地应放牧多少头牛? [Newton] There are two grasslands, A and B. The grass on the two grasslands is the same dense and fast. Twenty cows are known to have eaten grass on the grass for exactly 2 months; if they put 20 cows, then it is enough to eat for 4 months. If the area of ​​Grassland B is three times that of Grassland, then it is enough to make B grass grass enough to eat for six months. How many cattle should be grazing in B grassland?