Cattle grazing problem

, a square meadow is divided into exactly the same four pieces and the middle part of the shadow. It is known that the grass is evenly distributed at the beginning and is uniformly grown at a constant speed. But if the grass on one piece of ground is eaten up, No longer grow (because the grass roots are eaten). The farmer first took a herd of cows at the speed of the No. 1th grass. But if a piece of grass is eaten up, it will no longer grow (because the grass roots have been eaten). The farmer first took a herd of cattle to graze on the No. 1th lawn and ate all the grass on the 1th. The grass grows normally during the rest of the meadow). Then he let half the cows graze on the grass on the No. 2nd Lawn, and the other half graze on Meadow 3rd, and after 6 days, the grass on both grass was eaten. Finally, the old farmer put 3/5 of the cows on the shaded grass to graze, and the rest of the cows on the No. 4th grass, Finally found two grass on the grass at the same time to eat. If you start by letting the herd graze on the whole lawn, how many days will it take to finish the grass?

Cattle grazing problem