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+---
+epic: 3
+story: 3.5
+title: "Multi-Currency Arithmetic"
+status: draft
+---
+
+## Epic 3 — Currency & Cryptocurrency
+**Goal:** Real-time and historical currency conversion with 180+ fiat and 50+ crypto.
+
+### Story 3.5: Multi-Currency Arithmetic
+
+As a CalcPad user,
+I want to add and subtract amounts in different currencies within a single expression,
+So that I can calculate totals across currencies without manual conversions.
+
+**Acceptance Criteria:**
+
+**Given** the expression `$20 + €15 + £10 in BRL`
+**When** the engine evaluates it
+**Then** each amount is converted to BRL using current rates
+**And** the results are summed to produce a single BRL total
+
+**Given** the expression `$100 - €30`
+**When** the engine evaluates it
+**Then** `€30` is converted to USD (the currency of the left-hand operand)
+**And** the result is expressed in USD
+
+**Given** the expression `$50 + $30`
+**When** the engine evaluates it
+**Then** no conversion is needed (same currency)
+**And** the result is `$80`
+
+**Given** the expression `$20 + €15 + £10` with no explicit target currency
+**When** the engine evaluates it
+**Then** all amounts are converted to the currency of the first operand (USD)
+**And** the result is expressed in USD
+
+**Given** the expression `$20 * €15`
+**When** the engine evaluates it
+**Then** it returns a `CalcResult::Error` because multiplying two currency values is not meaningful
+**And** the error message is descriptive
+
+**Given** the expression `($100 + €50) * 2 in GBP`
+**When** the engine evaluates it
+**Then** the addition is performed first (converting EUR to USD), then multiplied by 2, then converted to GBP
+**And** the result is a single GBP value