This document discusses the law of equi-marginal utility, which states that a rational consumer will allocate their limited income across different goods in a way that equalizes the marginal utility per rupee spent. It provides an example showing how a consumer maximizes total utility by purchasing combinations of goods X and Y until their marginal utility per rupee is equal. The law has assumptions of rational behavior, full market knowledge, cardinal utility measurement, and substitutable divisible goods. Limitations are the assumptions of cardinal utility and rationality, as well as inability to apply to indivisible goods.