Ratio and proportion

A ratio is a comparison of two numbers. We generally separate the two numbers in the ratio with a colon (:) or write it as a fraction.

Example: Suppose we want to write the ratio of 8 and 12. We can write this either as 8:12 or as 8/12, and we say the ratio is eight to twelve.

We can simplify ratios the same as we simplify fractions.

Example: 8:12 is equivalent to 2:3

A proportion is one equality with a ratio on both sides.

Example: 

            
We say: 2 is to 3 as 8 is to 12

When one of the four numbers in a proportion is unknown, we have to use cross products to find the unknown number .

Examples:

Exercise: find out the value of x in each proportion:

Solutions: a) x =  45; b) x = 15; c) x = 15