A tractor and six cows can plow 1/5 every day.

A tractor and 16 cows can plow13 every day.

16-6= 10 cows, and1/3-1/5 = 2/15 cows can be raised every day.

Each cow can plow (2/15)/10 =1/75 every day.

Each tractor can plow1/5-1/75× 5 = 2/15 a day.

Two tractors can plow 2×2/ 15=4/ 15 in one day.

After ploughing in 2.5 days, plough 1/2.5=2/5 every day.

2/5-4/ 15=2/ 15.

Cattle are also needed: (2/15)/(1/75) =10 head.

2.

Suppose 1 100 million people consume 1 resource every year.

100 billion 100 year, need100×100 =10000.

8 billion in 300 years, 80× 300 = 24,000 copies are needed.

Annual renewable resources (24000-10000)/(300-100) = 70 copies.

Can feed 7 billion people.

3.

Suppose each sheep eats 1 serving of grass every day, then each cow eats 4 servings of grass every day.

16 cows graze for 20 days 16×4×20= 1280.

80 sheep grazed for 80× 12 = 960, 12 days.

Grass growing on grassland every day (1280-960)/(20-12) = 40 copies.

There are 960-40× 12=480 grasses on the grassland.

10 cattle and 60 sheep eat grass every day: 10×4+60= 100.

Edible: 480/( 100-40)=8 days.