A proportional relationship between two quantities is one in which the two quantities
vary directly with one and other. If one item is doubled, the other, related item is also
doubled. Because of this, it is also called a direct variation. The equations of such
relationships are always in the form y = mx , and when graphed produce a line that
passes through the origin. In this equation, m is the slope of the line, and it is also
called the unit rate, the rate of change, or the constant of proportionality of the function
