The general solution to this kind of problem is: regard the grass a sheep eats a day as 1, then there are:

(1) The grass that 25 sheep ate for 8 days was 25× 8 = 200.

(This 200 includes the original grass in the pasture and the grass that has grown for 8 days. )

(2) The grass eaten by 21sheep on 12 days is 2 1× 12 = 252.

(This 252 includes pasture grass and 12-day new grass. )

(3) 1 day: (252-200) ÷ (12-8) =13.

(4) The grassland is 25× 8-2 1× 8 = 32.

(5) Every day, the newly grown grass is enough for 13 sheep to eat, minus 13 from 15, and the remaining two sheep eat grass from the original pasture:

32 ÷ (15-13) = 32 ÷ 2 =16 (days)

Therefore, it takes 15 sheep to eat all the grass in the pasture within 16 days.