A ratio is a comparison of two numbers.

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

The ratio of two numbers a and b is the quotient

Sussan has a bag with 3 pens, 4 pencils, 7 felt tip pens, and 1 pencil sharpener. What is the ratio of pencils to pens?
Expressed as a fraction, with the numerator equal to the first quantity and the denominator equal to the second, the answer would be 4/3.
Two other ways of writing the ratio are 4 to 3, and 4:3.

Proportion

An equation in which two ratios are equal is called a proportion.

Two ratios and are a proportion when

Remember the two ratios are equals if their cross products are equal a·d = b·c

You can also use the cross product to find a missing number:

Find x to get a proportion .