diff --git a/src/main/java/PurchaseCalculator.java b/src/main/java/PurchaseCalculator.java
index 0ca34cb..78f8e90 100644
--- a/src/main/java/PurchaseCalculator.java
+++ b/src/main/java/PurchaseCalculator.java
@@ -1,6 +1,6 @@
 import java.math.BigDecimal;
 
-public class PurchaseCalculator {
+public class PurchaseCalculator implements TransactionCalculator {
     private BigDecimal purchasePrice;
     private BigDecimal quantity;
     
diff --git a/src/main/java/SaleCalculator.java b/src/main/java/SaleCalculator.java
index 476ed1f..f868729 100644
--- a/src/main/java/SaleCalculator.java
+++ b/src/main/java/SaleCalculator.java
@@ -1,6 +1,6 @@
 import java.math.BigDecimal;
 
-public class SaleCalculator {
+public class SaleCalculator implements TransactionCalculator{
     private BigDecimal purchasePrice;
     private BigDecimal salesPrice;
     private BigDecimal quantity;
diff --git a/src/main/java/TransactionCalculator.java b/src/main/java/TransactionCalculator.java
index dd17eef..0bafc6e 100644
--- a/src/main/java/TransactionCalculator.java
+++ b/src/main/java/TransactionCalculator.java
@@ -7,3 +7,4 @@ public interface TransactionCalculator {
     BigDecimal calculateTax();
     BigDecimal calculateTotal();
 }
+