The document discusses the law of equi-marginal utility, which states that consumers will allocate their expenditures across goods in such a way that the marginal utility per rupee spent is equal for all goods. It assumes utilities are independent and the marginal utility of money is constant. The law allows consumers to maximize satisfaction under their budget constraint. The document provides an example of a consumer with Rs. 5 to spend on goods A and B, each priced at Rs. 1, and calculates the marginal utility for spending units of money on each good until the marginal utilities are equal.