the quantitative relation between two amounts showing the number of times one value contains or is contained within the other.

ratio is a comparison of 2 quantities. 
How to Write a RatioDr. Pasture, a rural veterinarian, treats only 2 types of animals – cows and horses. Last week, she treated 12 cows and 16 horses.Part to Part Ratio:  What is the ratio of cows to horses that she treated?Simplify: 12:16 = 3:4Ratio Formats3:43 to 43/4For every 3 cows that Dr. Pasture treated, she treated 4 horses.

Part to Whole Ratio:  What is the ratio of cows that she treated to the total number of animals that she treated?Simplify: 12:30 = 2:5Ratio Formats2:52 to 52/5For every 5 animals that Dr. Pasture treated, 2 of them were cows. 
The quantitative relationship between two classes of objects.