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.