As the weather gradually gets colder, the grass on the pasture not only does not grow, but is reduced at a fixed rate.

Suppose the amount of grass a cow eats in one day is 1 portion.

20 cows eat 100 servings in 5 days, 15 cows eat 90 servings in 6 days, 100-90=10 (servings),

It means that the cold reduces the grass in the pasture by 10 servings in 1 day portion, that is, the cold is equivalent to 10 cows grazing.

From "the grass on the pasture can be eaten by 20 cows for 5 days", plus the 10 cows represented by "cold" are grazing at the same time,

So the original pasture Grass (20 + 10) × 5 = 150 (portions).

From

150÷10=15, we know that the original grass in the pasture can be eaten by 15 cows

In 10 days, the cold consumes 10 cows,< /p>

So, it can feed 5 cows for 10 days.