A proportion could be simply defined as the statement where two ratios are equal. It could be written in the two equal fractions like a/b = c/d; or you could use colon as well instead of backslash a: b = c: d. Take an example where twenty-five to twenty proportion could be simplified as five is to four.

If there are a few problems that include proportions then you can use cross products to test where two ratios are equal or forming a proportion or not. To find the cross product for a proportion, you should first multiple the outer values i.e. extreme values together then middle terms that are called as means. When product at both sides is equal then this is called as the true proportion in that case.

An equation is truly proportionate when elements a, b, c, and d, are in equal proportion or extremes are equal to mean terms in that case. The standard proportion formula in mathematic could be written as given below.

\[\large a:b::c:d\Rightarrow \frac{a}{b}=\frac{c}{d}\]

Logically, we can also use cross products to find a missing number in the proportion. It is frequently used for many real-world problems to find the actual output where unknown variable could be written as x and you have to find the value of unknown variable with cross multiplication technique.

You can also use inverse multiplying technique in a few cases to find the output. This is a topic of mathematics study for junior students and easy to understand with a little practice only.