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Ratios look like fractions and are related to fractions. Whereas a fraction describes a part of a whole, a ratio compares two parts.
The ratio of a to b may be written in different ways such as a to b, or a : b, or a ÷ b, or a / b where b not equal to 0
A proportion is a statement that show equality between two ratios .
The proportion a / b = c / d may be written in different ways such as a : b = c : d, or a : b :: c : d (read as 'a is to b as c is to d').
The terms on the outside, a and d, are called the extremes and the terms on the inside, b and c, are called the means.
Since a : b and c : d are ratios, b and d are nonzero.
On multiplying each side of the proportion by b d, the proportion becomes equivalent to the equation a d = b c.
In a proportion, the product of the extremes equals the product of the means.