The simple and compound interest formulas in isolation have limited usefulness for financial planning--until the concept of a regular payment is introduced. This article presents and discusses a simple investigation given to an advanced Year 10 mathematics class, with most students looking to study Mathematical Methods in Years 11 and 12. This investigation sought to fill a gap in the money and financial mathematics content of the "Australian Curriculum: Mathematics" by inviting students to explore the amortisation formula, develop students' understanding of borrowing and saving money, and provide a context for the discussion of financial decisions that students in this age group are making and likely to make in the future.