diff --git a/src/bank.ts b/src/bank.ts
index b5ee9bd..590ceb7 100644
--- a/src/bank.ts
+++ b/src/bank.ts
@@ -3,9 +3,13 @@ import { Money } from './money';
 
 export class Bank {
   reduce(source: Expression, to: string):Money {
-    return source.reduce(to);
+    return source.reduce(this, to);
   }
 
-  addRate(from: string, to: string, rate: number) { 
+  addRate(from: string, to: string, rate: number) {
+  }
+
+  rate(from: string, to: string): number {
+    return from === 'CHF' && to === 'USD' ? 2 : 1
   }
 }
diff --git a/src/expression.ts b/src/expression.ts
index 120a2ba..cf9f153 100644
--- a/src/expression.ts
+++ b/src/expression.ts
@@ -1,5 +1,6 @@
+import { Bank } from "./bank";
 import { Money } from "./money";
 
 export interface Expression{
-  reduce(to: string): Money;
+  reduce(bank: Bank, to: string): Money;
 }
diff --git a/src/money.ts b/src/money.ts
index 6916583..ee13485 100644
--- a/src/money.ts
+++ b/src/money.ts
@@ -1,3 +1,4 @@
+import { Bank } from "./bank";
 import { Expression } from "./expression";
 import { Sum } from "./sum";
 
@@ -33,8 +34,8 @@ export class Money implements Expression {
     return new Sum(this, addend);
   }
 
-  reduce(to: string): Money {
-    const rate = this.currency === 'CHF' && to === 'USD' ? 2 : 1;
-    return new Money(this.amount / rate, to)
+  reduce(bank: Bank,to: string): Money {
+    const rate = bank.rate(this.currency, to);
+    return new Money(this.amount / rate, to);
   }
 }
diff --git a/src/sum.ts b/src/sum.ts
index 8c3914f..59191d9 100644
--- a/src/sum.ts
+++ b/src/sum.ts
@@ -1,3 +1,4 @@
+import { Bank } from "./bank";
 import { Expression } from "./expression";
 import { Money } from "./money";
 
@@ -5,7 +6,7 @@ export class Sum implements Expression {
   constructor(public augend: Money, public addend: Money) {
   }
 
-  reduce(to: string):Money {
+  reduce(bank: Bank, to: string):Money {
     const amount :number = this.augend.amount + this.addend.amount;
     return new Money(amount, to);
   }