21. (7th grade) (Newton's problem) Grass grows at the same rate and density everywhere in the corner. It is known that 70 cows would eat it in 24 days, and 30 cows in 60 days. How many cows would eat all the grass in 96 days? (It is assumed that the cows eat the grass uniformly).
21. In one day, $x$ amount of grass grows. If the initial amount of grass is taken as 1, then over 24 days, the cows eat $1+24 x$ amount of grass, and one cow eats $\frac{1+24 x}{24 \cdot 70}$ amount of grass per day. But according to the second condition, a cow eats $\frac{1+60 x}{30 \cdot 60}$ amount of grass per day. By equating the first and second results, we get $x=\frac{1}{480}$. A cow eats $\frac{1}{1600}$ of the initial amount of grass per day. In 96 days, 20 cows will eat all the grass.