Proportions and percentages are used to report quantities like interest rates, inflation rates, and unemployment rates. A percentage is a proportion multiplied by 100. For example, if a $3,000 investment earns 7.6% annual interest, the interest income would be $3,000 * 0.076 = $288. In economics, percentage changes are often used to measure how quantities change over time. The percentage change formula takes the proportional change and multiplies it by 100. Non-linear relationships can be modeled using quadratic or logarithmic functions to capture effects like diminishing returns. Logarithmic and semi-logarithmic models are commonly used to model elasticities in economics.