In this chapter, we study the mathematics of finance: interest, investments, and loans. There are some very deep mathematical ideas related to finance, and a full study is well beyond the scope of this book. However, we discuss interest, APR and APY, equity, and variants such as add-on loans. Simple and compound interest are discussed and regular savings models are investigated. We generalize to continuous compounding and exponential growth, and show how the mathematics of finance can be applied to the growth of populations and to radioactive decay.