Complete problem solving ideas:

It is assumed that the daily grazing amount of each cow is 1, and the grazing amount of 27 cows in the first 6 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 grazing amount of 27 cows in six days is 162, and the sum of newly grown grasses in these six days is 15×6=90, we can know that the original grazing amount of this pasture is 162-90=72.

There are 15 cows to eat in the pasture every day, 2 1 5 cows are allowed to eat newly grown grass every day, and the remaining 21-15 = 6 (heads) only eat the original grass. 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.