She should buy five sweaters.
A rational consumer would purchase a good as long as marginal benefit is equal or greater than marginal cost.
Marginal cost is the additional cost generated by producing an additional unit of output. Marginal benefit is the benefit derived from consuming one extra unit of a good.
The marginal benefit of the first five sweaters is greater than the marginal cost. Thus, she should buy five sweaters.
To learn more about marginal cost, please check: brainly.com/question/26246533
#SPJ1
