In direct proportion, as the first variable increases (decreases), the second variable also increases (decreases). In mathematical statements, it can be expressed as y = kx. This reads as “y varies directly as x” or “y is directly proportional as x” where k is constant in the equation
In this equation, y and x are the two directly proportional quantities and k is a proportionality constant that tells you exactly how they are related (like 10 cents/piece above). On a graph, a direct proportion will look like a straight line and the slope of the line will be equal to the constant k
