Let Yi eat 1 serving of grass every day. 15 cows ate it for 8 days, 1 head * * * ate it 120, 8 cows ate it for 20 days, * * * ate it 160, with a difference of 40, because the grass grew in 12 days. You can calculate the daily growth of grass 10/3 (I omitted the calculation step, I think it is relatively simple, you can understand). Because each cow needs to eat 1 serving every day (previously set), 10/3 serving of grass growing every day needs 10/3 cows to eat, and other cows can be said to be eating the original grassland. So as to calculate the original grassland (

(15- 10/3) times 8)=280/3 copies. So what a group of cows eat in 28 days is 280/3+28 times 10/3=560/3. Divide 560/3 by 28 and then divide it by 1 the number of servings each cow eats every day, and you can get the answer 20/3 cows.