The school library bought three hardcover books and five paperback books "Chinese Dictionary", which cost 27.8 yuan, which can be regarded as buying six paperback books and five paperback books 27.8+3=30.8 yuan.
30.8 (6+5) per paperback book = 2.8 yuan.
Each hardcover book (27.8-2.5×5)÷3=5. 1 yuan.
If the Yi people in A Niu eat three times as much grass as a sheep, it means that one cow eats the same grass as three sheep.
Three cows and eight sheep eat 42.5 kilograms of grass a day, which can be regarded as 3×3=9 sheep and 8 sheep eat 42.5 kilograms of grass. Each sheep eats grass 42.5÷(3+8)=2.5 kg.