A proportion is a name we give to a

A proportion is a name we give to a statement that two ratios are equal. It can be written in two ways:
• two equal fractions,
or,
• using a colon, a:b = c:d
When two ratios are equal, then the cross products of the ratios are equal.
That is, for the proportion, a:b = c:d , a x d = b x c