The variations between quantities are expressed in terms of proportionality. These variations can be either direct or indirect or joint variations.
When two quantities vary directly, they can be related using a nonzero number k, where k is called the constant of proportionality.
If a quantity x varies directly with the quantity y, then the variation expressed using k is
x = ky.
Therefore, if x and y are two quantities, then y is directly proportional to x if there is a nonzero number k such that y = kx.