假设1只羊1天吃1份草,7天吃:1*7=7份,
2只羊3天吃:1*2*3=6份.
草7天比3天多:7-6=1份,草每天生长:1/(7-3)=1/4份.
草地原有草:7-7*1/4=21/4份.
3只羊能吃:(21/4)/(1*3-1/4)=21/11天.
假设1只羊1天吃1份草,7天吃:1*7=7份,
2只羊3天吃:1*2*3=6份.
草7天比3天多:7-6=1份,草每天生长:1/(7-3)=1/4份.
草地原有草:7-7*1/4=21/4份.
3只羊能吃:(21/4)/(1*3-1/4)=21/11天.