The Yangtze Farm has a pasture, where grass grows evenly every day. If 24 cows are grazed on the pasture, the grass will be eaten up in 6 days; if only 21 cows are grazed, it will take 8 days to finish the grass. How many days will it take to finish the grass if 36 cows are grazed?
【Analysis and Solution】
The problem of cows eating grass.
Assume 1 cow eats 1 portion per day;
24 cows eat $24 \times 6=144$ portions in 6 days;
21 cows eat $21 \times 8=168$ portions in 8 days;
Grass grows $(168-144) \div(8-6)=12$ portions per day;
The original amount of grass in the pasture is $(24-12) \times 6=72$ or $(21-12) \times 8=72$ portions; if 36 cows are grazed, $72 \div(36-12)=3$ days will be needed to finish the grass.