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+---
+epic: 6
+story: 6.4
+title: "Financial Functions"
+status: draft
+---
+
+## Epic 6 — Advanced Math & Functions
+**Goal:** Scientific, financial, and power-user math.
+
+### Story 6.4: Financial Functions
+
+As a **CalcPad user**,
+I want to use natural-language financial expressions for compound interest and mortgage calculations,
+So that I can quickly answer personal finance questions without looking up formulas.
+
+**Acceptance Criteria:**
+
+**Given** the user writes `$5000 after 3 years at 7.5%`
+**When** the engine evaluates (compound interest, annual compounding by default)
+**Then** the result is approximately `$6,211.94`
+
+**Given** the user writes `mortgage $350000 at 6.5% for 30 years`
+**When** the engine evaluates (standard monthly payment amortization)
+**Then** the result is approximately `$2,212.24` per month
+
+**Given** the user writes `$10000 after 5 years at 5% compounded monthly`
+**When** the engine evaluates
+**Then** the result is approximately `$12,833.59`
+
+**Given** the user writes `mortgage $200000 at 4% for 15 years`
+**When** the engine evaluates
+**Then** the result is approximately `$1,479.38` per month
+
+**Given** the user writes a compound interest expression with 0% interest
+**When** the engine evaluates
+**Then** the result equals the principal (no growth)
+
+**Given** the user writes a mortgage expression with 0% interest
+**When** the engine evaluates
+**Then** the monthly payment is the principal divided by the total number of months
+
+**Given** the user writes `$5000 after 0 years at 7.5%`
+**When** the engine evaluates
+**Then** the result is `$5,000` (the principal unchanged)
+
+**Given** the user writes a financial expression with missing or invalid parameters (e.g., `mortgage at 5%`)
+**When** the engine evaluates
+**Then** an error is displayed indicating the required format or missing parameters