diff --git a/gradebook/src/GradeBook/Book.cs b/gradebook/src/GradeBook/Book.cs
index e2f704b..c4c438d 100644
--- a/gradebook/src/GradeBook/Book.cs
+++ b/gradebook/src/GradeBook/Book.cs
@@ -18,7 +18,20 @@ namespace GradeBook
             set;
         }
     }
-    public class Book : NamedObject
+
+    public abstract class Book : NamedObject
+    {
+        // Abstract class used to act as entrypoint for other class, so output 
+        // can change based on given input. At this level, we cannot provide implementation
+        // Let derived class to determine details.
+
+        public Book(string name) : base(name)
+        {
+
+        }
+        public abstract void AddGrade(double grade);
+    }
+    public class InMemoryBook : Book
     {
         private List<double> grades;
 
@@ -26,13 +39,13 @@ namespace GradeBook
 
         private string name;
 
-        public Book(string name) : base(name)
+        public InMemoryBook(string name) : base(name)
         {
             Name = name;
             grades = new List<double>();
         }
 
-        public void AddGrade(double grade)
+        public override void AddGrade(double grade)
         {
             if (grade <= 100 && grade >= 0)
             {
diff --git a/gradebook/src/GradeBook/Program.cs b/gradebook/src/GradeBook/Program.cs
index 2134bdd..96c4ad9 100644
--- a/gradebook/src/GradeBook/Program.cs
+++ b/gradebook/src/GradeBook/Program.cs
@@ -8,11 +8,23 @@ namespace GradeBook
         static void Main(string[] args)
         {
 
-            var book = new Book("Jason's Grade Book");
+            var book = new InMemoryBook("Jason's Grade Book");
             book.GradeAdded += OnGradeAdded;
 
             string input;
 
+            input = EnterGrades(book);
+
+            var stats = book.GetStatistics();
+
+            Console.WriteLine($"The lowest grade is {stats.lowGrade}");
+            Console.WriteLine($"The highesst grade is {stats.highGrade}");
+            Console.WriteLine($"The average grade is {stats.Average:N1}");
+        }
+
+        private static string EnterGrades(Book book)
+        {
+            string input;
             while (true)
             {
                 Console.WriteLine("Give input: ");
@@ -21,12 +33,13 @@ namespace GradeBook
                 {
                     Console.WriteLine("Receiving termination signal; Termiate program");
                     break;
-                } else
+                }
+                else
                 {
                     try
                     {
                         Console.WriteLine($"Received: {input}");
-                        book.AddGrade(Convert.ToDouble(input));   
+                        book.AddGrade(Convert.ToDouble(input));
                         // book.AddGrade('A');
                     }
                     catch (ArgumentException ex)
@@ -45,11 +58,7 @@ namespace GradeBook
                 }
             }
 
-            var stats = book.GetStatistics();
-
-            Console.WriteLine($"The lowest grade is {stats.lowGrade}");
-            Console.WriteLine($"The highesst grade is {stats.highGrade}");
-            Console.WriteLine($"The average grade is {stats.Average:N1}");
+            return input;
         }
 
         static void OnGradeAdded(object sender, EventArgs e)
diff --git a/gradebook/test/GradeBook.Tests/BookTests.cs b/gradebook/test/GradeBook.Tests/BookTests.cs
index f329f85..5d49b84 100644
--- a/gradebook/test/GradeBook.Tests/BookTests.cs
+++ b/gradebook/test/GradeBook.Tests/BookTests.cs
@@ -9,7 +9,7 @@ namespace GradeBook.Tests
         public void BookCalculateAnAverageGrade()
         {
             // arrange
-            var book = new Book("");
+            var book = new InMemoryBook("");
             book.AddGrade(89.1);
             book.AddGrade(90.5);
             book.AddGrade(77.3);
diff --git a/gradebook/test/GradeBook.Tests/TypeTests.cs b/gradebook/test/GradeBook.Tests/TypeTests.cs
index f98c9a1..de3a849 100644
--- a/gradebook/test/GradeBook.Tests/TypeTests.cs
+++ b/gradebook/test/GradeBook.Tests/TypeTests.cs
@@ -77,9 +77,9 @@ namespace GradeBook.Tests
             Assert.Equal(book1.Name, "New Name");
         }
 
-        private void GetBookSetName(ref Book book, string name)
+        private void GetBookSetName(ref InMemoryBook book, string name)
         {
-            book = new Book(name);
+            book = new InMemoryBook(name);
             book.Name = name;
         }
 
@@ -92,9 +92,9 @@ namespace GradeBook.Tests
             Assert.Equal(book1.Name, "Book1");
         }
 
-        private void GetBookSetName(Book book, string name)
+        private void GetBookSetName(InMemoryBook book, string name)
         {
-            book = new Book(name);
+            book = new InMemoryBook(name);
             book.Name = name;
         }
 
@@ -107,7 +107,7 @@ namespace GradeBook.Tests
             Assert.Equal(book1.Name, "New Name");
         }
 
-        private void SetName(Book book, string name)
+        private void SetName(InMemoryBook book, string name)
         {
             book.Name = name;
         }
@@ -133,9 +133,9 @@ namespace GradeBook.Tests
         }
 
 
-        private Book GetBook(string name)
+        private InMemoryBook GetBook(string name)
         {
-            return new Book(name);
+            return new InMemoryBook(name);
         }
     }
 }