【Question 22】
There is a uniformly growing grassland. If 20 cows are grazed, it will take 60 days to finish all the grass. If 30 cows are grazed, it will take 35 days to finish all the grass. Now, 6 cows are grazed on this grassland, and after one month, 10 more cows come to the grassland. Then, it will take another $\qquad$ days to finish all the grass on the grassland.
【Analysis and Solution】
The cow eating grass problem.
Assume 1 cow eats 1 portion of grass per day;
20 cows eat $20 \times 60=1200$ portions of grass in 60 days;
30 cows eat $30 \times 35=1050$ portions of grass in 35 days;
The grass grows $(1200-1050) \div(60-35)=6$ portions per day;
The original amount of grass is $(20-6) \times 60=840$ or $(30-6) \times 35=840$ portions;
Now, 6 cows are grazing on this grassland, and after one month, 10 more cows join. It will take another $840 \div 10=84$ days to finish all the grass on the grassland.