(1) The forage for 27 cows in 6 days is: 27×6= 162, (this 162 includes the original grass in the pasture and the grass newly grown in 6 days. )

(2) The pasture eaten by 23 cows in 9 days was 23×9=207.

(This 207 includes the original grass in the pasture and the grass that has grown for 9 days. )

(3) 1 day: (207-162) ÷ (9-6) =15.

(4) The grassland is 27×6- 15×6=72.

(5) Feed new grass to 15 cows every day, subtract 2 1 cow from 15 cows, and the remaining 6 cows eat grass from the original pasture;

72 ÷ (21-15) = 72 ÷ 6 =12 (days)

Answer: It takes 2 1 2 days for a cow to eat all the grass on the pasture.