Four cows and Sanma eat 90kg of grass every day, multiplied by 2, and eight cows and six horses eat 180kg of grass every day. Subtract it from "eight cows and two horses eat 140kg grass every day", and you can get

Four horses eat 40 kilograms of grass every day, so each horse eats 40÷4= 10 kilograms of grass every day.

The daily grazing amount of each cow is (90-3× 10)÷4= 15 kg.