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+---
+epic: 6
+story: 6.5
+title: "Proportions / Rule of Three"
+status: draft
+---
+
+## Epic 6 — Advanced Math & Functions
+**Goal:** Scientific, financial, and power-user math.
+
+### Story 6.5: Proportions / Rule of Three
+
+As a **CalcPad user**,
+I want to solve proportion problems using natural language,
+So that I can quickly compute unknown values in ratios without manually setting up equations.
+
+**Acceptance Criteria:**
+
+**Given** the user writes `3 is to 6 as what is to 10?`
+**When** the engine evaluates
+**Then** the result is `5`
+
+**Given** the user writes `3 is to 6 as 5 is to what?`
+**When** the engine evaluates
+**Then** the result is `10`
+
+**Given** the user writes `10 is to 5 as 20 is to what?`
+**When** the engine evaluates
+**Then** the result is `10`
+
+**Given** the user writes `0 is to 5 as what is to 10?`
+**When** the engine evaluates
+**Then** the result is `0`
+
+**Given** the user writes `3 is to 0 as 5 is to what?`
+**When** the engine evaluates
+**Then** an error is displayed (division by zero in the proportion)