回複：麵試問題請教

It took me about 5 -10 mins. The answer is never. The difficult part is how to define the rate of cow eating and grass growing. I defined the total amount of grass is 1 from the begining. The daily eating rate is 1/n, and the daily growing rate is 1/x. so you get:
3/n * 3 = 1 + 3/x; 2/n *6 = 1 + 6/x; => n=6 and x=6