A consumer of a single commodity is in equilibrium when

A consumer of a single commodity is in equilibrium when he/she can equate his/her marginal utility with the price of the commodity. This means that the consumer has maximized the satisfaction derived from consuming the last unit of the commodity with the cost incurred in acquiring it. At this point, the consumer will not be willing to buy more of the commodity at the prevailing price because the additional satisfaction derived from it would be less than the cost of acquiring it. On the other hand, if the price of the commodity decreases, the consumer will increase his/her demand for it until the marginal utility of the last unit purchased is equal to the new price. Conversely, if the price increases, the consumer will decrease his/her demand for it until the marginal utility of the last unit purchased is equal to the new price.