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Grazing problem
Let a cow eat grass 1 day is 1.

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.