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Simple interest: I = P × r × t. Total = P + I. Compound interest: A = P × (1 + r/n)^(nt), where n is compounding periods per year. Daily compounding at 5% APR yields ~5.13% APY. Savings accounts quote APY; loans quote APR—compare like-for-like. Rule of 72: years to double ≈ 72 / annual rate%.
Simple interest: I = P × r × t (annual rate, time in years). Does not compound.