The document defines interest and discusses how to calculate simple interest, maturity value, and how to convert between annual and monthly interest rates. It provides examples of calculating interest for various scenarios like loans, investments, and credits. Calculating interest involves determining the principal (P), interest rate (R) as a decimal, and time (T) in years or months. The maturity value (M) is calculated as the principal plus interest. Examples show how to determine interest, rates, principal amounts and maturity values for real-life situations.