How many cow and chickens if there are 9 heads and 18 legs

1 / 7 chickens and 1 cow >>> 7 x 2 + 1 x 4 = 18 (legs)

2 / 5 chickens and 2 cows >>> 5 x 2 + 2 x 4 = 18 (legs)

3 / 3 chickens and 3 cows >>> 3 x 2 + 3 x 4 = 18 (legs)

4 / 1 chicken and 4 cows >>> 1 x 2 + 4 x 4 = 18 (legs)

There are multiple solutions. One is 7 chicken and 1 cow.