This kind of problem is called: the complete solution of Newton's problem: suppose that the daily grazing amount of each cow is 1, and the grazing amount of 27 cows in the first six days is 27× 6 =162; The grazing amount of 23 cows in 9 days is 23×9=207. The difference between 207 and 162 is grass newly grown in (9-6) days, so the amount of grass newly grown in the pasture every day is (207-162) ÷ (9-6) =15. Because the amount of grass eaten by 27 cows in 6 days is 162, they are newly grown in these 6 days. Therefore, it can be seen that the original grazing amount of this pasture is 162-90=72. The newly grown grass in the pasture is enough for 15 cows to eat for one day. Let 2 1 5 cows eat the newly grown grass every day, and the remaining 21-15 = 6 (head). So the grass on the pasture is enough to eat 72÷6= 12 (days), that is, the grass on this pasture is enough to eat 2 1 cow 12 days. Comprehensive formula: [27× 6-(23× 9-27× 6) ÷ (9-6) ÷ [21-(23× 9-27× 6) ÷ (9-6)] = 65438.