diff --git a/refactoring/src/main/java/com/example/refactoring/_06_mutable_data/_21_replace_derived_variable_with_query/Discount.java b/refactoring/src/main/java/com/example/refactoring/_06_mutable_data/_21_replace_derived_variable_with_query/Discount.java
new file mode 100644
index 00000000..693c811c
--- /dev/null
+++ b/refactoring/src/main/java/com/example/refactoring/_06_mutable_data/_21_replace_derived_variable_with_query/Discount.java
@@ -0,0 +1,20 @@
+package com.example.refactoring._06_mutable_data._21_replace_derived_variable_with_query;
+
+public class Discount {
+
+    private double discount;
+
+    private double baseTotal;
+
+    public Discount(double baseTotal) {
+        this.baseTotal = baseTotal;
+    }
+
+    public double getDiscountedTotal() {
+        return this.baseTotal - this.discount;
+    }
+
+    public void setDiscount(double number) {
+        this.discount = number;
+    }
+}
diff --git a/refactoring/src/main/java/com/example/refactoring/_06_mutable_data/_21_replace_derived_variable_with_query/ProductionPlan.java b/refactoring/src/main/java/com/example/refactoring/_06_mutable_data/_21_replace_derived_variable_with_query/ProductionPlan.java
new file mode 100644
index 00000000..1f16ef6a
--- /dev/null
+++ b/refactoring/src/main/java/com/example/refactoring/_06_mutable_data/_21_replace_derived_variable_with_query/ProductionPlan.java
@@ -0,0 +1,17 @@
+package com.example.refactoring._06_mutable_data._21_replace_derived_variable_with_query;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class ProductionPlan {
+
+    private List<Double> adjustments = new ArrayList<>();
+
+    public void applyAdjustment(double adjustment) {
+        this.adjustments.add(adjustment);
+    }
+
+    public double getProduction() {
+        return this.adjustments.stream().mapToDouble(Double::valueOf).sum();
+    }
+}
diff --git a/refactoring/src/main/java/com/example/refactoring/_06_mutable_data/_21_replace_derived_variable_with_query/before/Discount.java b/refactoring/src/main/java/com/example/refactoring/_06_mutable_data/_21_replace_derived_variable_with_query/before/Discount.java
new file mode 100644
index 00000000..ea5a6393
--- /dev/null
+++ b/refactoring/src/main/java/com/example/refactoring/_06_mutable_data/_21_replace_derived_variable_with_query/before/Discount.java
@@ -0,0 +1,22 @@
+package com.example.refactoring._06_mutable_data._21_replace_derived_variable_with_query.before;
+
+public class Discount {
+
+    private double discountedTotal;
+    private double discount;
+
+    private double baseTotal;
+
+    public Discount(double baseTotal) {
+        this.baseTotal = baseTotal;
+    }
+
+    public double getDiscountedTotal() {
+        return this.discountedTotal;
+    }
+
+    public void setDiscount(double number) {
+        this.discount = number;
+        this.discountedTotal = this.baseTotal - this.discount;
+    }
+}
diff --git a/refactoring/src/main/java/com/example/refactoring/_06_mutable_data/_21_replace_derived_variable_with_query/before/ProductionPlan.java b/refactoring/src/main/java/com/example/refactoring/_06_mutable_data/_21_replace_derived_variable_with_query/before/ProductionPlan.java
new file mode 100644
index 00000000..c7fb30aa
--- /dev/null
+++ b/refactoring/src/main/java/com/example/refactoring/_06_mutable_data/_21_replace_derived_variable_with_query/before/ProductionPlan.java
@@ -0,0 +1,19 @@
+package com.example.refactoring._06_mutable_data._21_replace_derived_variable_with_query.before;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class ProductionPlan {
+
+    private double production;
+    private List<Double> adjustments = new ArrayList<>();
+
+    public void applyAdjustment(double adjustment) {
+        this.adjustments.add(adjustment);
+        this.production += adjustment;
+    }
+
+    public double getProduction() {
+        return this.production;
+    }
+}
diff --git a/refactoring/src/test/java/com/example/refactoring/_06_mutable_data/_21_replace_derived_variable_with_query/DiscountTest.java b/refactoring/src/test/java/com/example/refactoring/_06_mutable_data/_21_replace_derived_variable_with_query/DiscountTest.java
new file mode 100644
index 00000000..75c6bd4d
--- /dev/null
+++ b/refactoring/src/test/java/com/example/refactoring/_06_mutable_data/_21_replace_derived_variable_with_query/DiscountTest.java
@@ -0,0 +1,18 @@
+package com.example.refactoring._06_mutable_data._21_replace_derived_variable_with_query;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class DiscountTest {
+
+    @Test
+    void discount() {
+        Discount discount = new Discount(100);
+        assertEquals(100, discount.getDiscountedTotal());
+
+        discount.setDiscount(10);
+        assertEquals(90, discount.getDiscountedTotal());
+    }
+
+}
\ No newline at end of file
diff --git a/refactoring/src/test/java/com/example/refactoring/_06_mutable_data/_21_replace_derived_variable_with_query/ProductionPlanTest.java b/refactoring/src/test/java/com/example/refactoring/_06_mutable_data/_21_replace_derived_variable_with_query/ProductionPlanTest.java
new file mode 100644
index 00000000..e7de8ef5
--- /dev/null
+++ b/refactoring/src/test/java/com/example/refactoring/_06_mutable_data/_21_replace_derived_variable_with_query/ProductionPlanTest.java
@@ -0,0 +1,17 @@
+package com.example.refactoring._06_mutable_data._21_replace_derived_variable_with_query;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class ProductionPlanTest {
+
+    @Test
+    void production() {
+        ProductionPlan productionPlan = new ProductionPlan();
+        productionPlan.applyAdjustment(10);
+        productionPlan.applyAdjustment(20);
+        assertEquals(30, productionPlan.getProduction());
+    }
+
+}
\ No newline at end of file