Grazing Grass

Relevant wiki: Ratio, Rate and Proportion - Problem Solving

Nine cows eat 1 field/hr. So three cows eat 1/3 field/hr. So in nine hours they can eat three fields, and 9:3 = 3:1

Since we are given 9 cows eat 9 fields in 9 hours, we have the equation $\frac{9c}{9f} = 9h$ (where c stands for cows, f stands for fields, and h stands for hours). After simplification, we have that $\frac{c}{f} = 9h$ . So, to find the time for 3 cows to eat 3 fields, we multiply both the denominator and the numerator by 3, to get $\frac{c \times 3}{f \times 3} = \frac{3c}{3f} = 9h$ . So 3 cows eat 3 fields in 9 hours.

You can also apply, direct proportion here.