Variation and proportion are defined to be the relationship between two or more variables with regard to a constant of proportionality.

The traditional notation for direct proportionality is $x \propto y$ or, if using regular equality notation, $x = ky$ .

Here, $k$ denotes the constant of proportionality.

Similarly, the traditional notation for inverse proportionality is $x \propto 1/y$ or, with regular equality, $x = k/y$ .

For direct proportionality, to find the value of an unknown $x$ or $y$ , you may use the formula: $y_{1}/x_{1} = y_{2}/x_{2}$

Similarly, for inverse proportion it would be: $x_{1}/y_{1} = y_{2}/x_{2}$