This article proposes an in-depth mathematics analysis of simple and compound interest through an exploration of the mathematical structures underlying these concepts. Financial numeracy concepts (simple and compound interest, interest rates, interest period, present and future value, etc.) have been added to the mathematics curriculum of the Canadian province of Quebec in 2016. Yet, little is known with regards to mathematics teachers' knowledge of financial concepts. We mobilized the relational paradigm framework in the context of simple and compound interest to construct an assessment instrument of teacher knowledge. Based on 36 teachers' responses to the questionnaire, we observed that the majority of teachers did not use mathematical structures in problem solving; they were not able to make sense of simple and compound interest in ways that do not involve standard formulas. Such results indicate opportunities for professional development in regard to secondary mathematics teachers' knowledge of mathematics in the context of simple and compound interest situations.