Five acres of land can feed 10 cows for 30 days.
Then 15 mu of land can feed 30 cows for 30 days.
30 cows eat grass for 30 days;
30×30=900 copies
28 cows graze for 45 days:
28×45= 1260 copies
Difference: 1260-900=360 copies.
These 360 copies are grasses that have grown on 15 mu of land for 45-30 = 15 days.
So 15 mu of land is planted with grass every day:
360/ 15=24 copies
15 mu of land used to have grass;
900-24×30= 180 copies
So, 24 acres of land,
There is grass:
24/ 15 * 180=288 copies.
Planting grass every day:
24/ 15 *24=38.4 copies
After eating for 80 days, I ate grass:
38.4×80+288=3360 copies
Eat grass on average every day:
3360/80=42 copies
So 24 acres of land can supply 42 cows for 80 days.