A ratio tells us relative size. It does not usually tell us the actual amounts. A proportional model goes further: it says what stays unchanged when a quantity varies. Confusing these two ideas can give a neat calculation with an unjustified answer. This lesson develops four connected skills: comparing like quantities in common units; recovering actual shares from a total or a difference; combining ratios through a common quantity; and deciding whether a constant quotient or a constant product describes a situation. You will also learn why a straight graph with a fixed offset is not a direct-proportion graph.