The constant of proportionality is a fixed number that shows the relationship between two proportional quantities). It is also known as the coefficient of proportionality or the unit rate. In a direct proportion, the constant of proportionality is the ratio of the two proportional quantities, which can be expressed as k = y/x, where y and x are the two quantities that are directly proportional to each other. Once the constant of proportionality is known, an equation representing the directly proportional relationship between x and y can be found, which is y = kx. In an inverse proportion, the equation for the indirect proportionality is y = k/x, where k is also the constant of proportionality. The constant of proportionality helps in solving equations involving ratios and proportions.