The ratio is basically finding the quotient of two numbers. If you want to find the ratio of horses to cows, divide the number of horses to that of cows.
Ratio = 18/27
When expressing it to its simplest form, make sure it is reduced. 18 and 27 are both factors of 9. So,
Ratio = 9*2/9*3
Cancelling out 9, the ratio at its simplest form is: 2/3