diff --git a/DesignPatternsSolution/Execise.Tests/Execise.Tests.csproj b/DesignPatternsSolution/Execise.Tests/Execise.Tests.csproj
index 71e68ac..252ccb3 100644
--- a/DesignPatternsSolution/Execise.Tests/Execise.Tests.csproj
+++ b/DesignPatternsSolution/Execise.Tests/Execise.Tests.csproj
@@ -8,7 +8,7 @@
     <AppDesignerFolder>Properties</AppDesignerFolder>
     <RootNamespace>Execise.Tests</RootNamespace>
     <AssemblyName>Execise.Tests</AssemblyName>
-    <TargetFrameworkVersion>v4.5.2</TargetFrameworkVersion>
+    <TargetFrameworkVersion>v4.5</TargetFrameworkVersion>
     <FileAlignment>512</FileAlignment>
     <ProjectTypeGuids>{3AC096D0-A1C2-E12C-1390-A8335801FDAB};{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}</ProjectTypeGuids>
     <VisualStudioVersion Condition="'$(VisualStudioVersion)' == ''">10.0</VisualStudioVersion>
diff --git a/DesignPatternsSolution/Execise.Tests/UnitTests.cs b/DesignPatternsSolution/Execise.Tests/UnitTests.cs
index 0a3b80e..1570aec 100644
--- a/DesignPatternsSolution/Execise.Tests/UnitTests.cs
+++ b/DesignPatternsSolution/Execise.Tests/UnitTests.cs
@@ -10,20 +10,25 @@ public class UnitTests
         [TestMethod]
         public void LibraryShouldBeSingleton()
         {
-            //check that when you create a Library instance second time, 
-            //you get exactly the same instance as for a very first time
+            Library lib1 = Library.GetInstance();
+            Library lib2 = Library.GetInstance();
+            Assert.ReferenceEquals(lib1, lib2);
         }
 
         //test that an book was registered successfully by checking the returned Id value is not -1
         [TestMethod]
         public void BookShouldRegister()
         {
+            Book book = new Book("John Smith", "Adventures in Coding", 2017, 1, new Exercise.RegistrationApi());
+            Assert.IsTrue(RegistrationRepository.Register(book) > -1);
         }
 
         //test that an customer was registered successfully by checking the returned Id value is not -1
         [TestMethod]
         public void CustomerShouldRegister()
         {
+            Customer customer = new Customer("Jane Doe", "386 Broadway", new Exercise.RegistrationApi());
+            Assert.IsTrue(RegistrationRepository.Register(customer) > -1);
         }
 
         //test that a book can be borrowed
@@ -32,7 +37,11 @@ public void CanBorrowBook()
         {
             //create a borrowable book with available amount more than one. 
             //Run BorrowOne method of the BookBorrowable instance. Check that total amount was reduced by one.
-            
+            int totalAmount = 10;
+            Book book = new Book("John Smith", "Adventures in Coding", 2017, totalAmount, new Exercise.RegistrationApi());
+            BookBorrowable borrowable = new BookBorrowable(null, book);
+            borrowable.BorrowOne();
+            Assert.IsTrue(book.AvailableAmount == (totalAmount - 1));
         }       
     }
 }
diff --git a/DesignPatternsSolution/Exercise/BookBorrowable.cs b/DesignPatternsSolution/Exercise/BookBorrowable.cs
index 21bd9cf..d84f060 100644
--- a/DesignPatternsSolution/Exercise/BookBorrowable.cs
+++ b/DesignPatternsSolution/Exercise/BookBorrowable.cs
@@ -1,7 +1,41 @@
 namespace Exercise
 {
-    public class BookBorrowable 
+    public abstract class Borrowable
     {
-       
+        public virtual void BorrowOne() {}
+
+        protected LibObject LibObject;
+        public Borrowable(LibObject libObject)
+        {
+            LibObject = libObject;
+        }
+    }
+
+    public class BookBorrowable : Borrowable
+    {
+        private Borrowable borrowable;
+
+        public BookBorrowable(Borrowable borrowable, Book book)
+            : base(book)
+        {
+            this.borrowable = borrowable;
+        }
+
+        public override void BorrowOne()
+        {
+            if (borrowable != null)
+            {
+                borrowable.BorrowOne();
+            }
+
+            if (LibObject.AvailableAmount > 0)
+            {
+                LibObject.AvailableAmount--;
+            }
+            else
+            {
+                throw new System.Exception("No books left to borrow.");
+            }
+        }
     }
 }
diff --git a/DesignPatternsSolution/Exercise/Exercise.csproj b/DesignPatternsSolution/Exercise/Exercise.csproj
index d2cc582..f3315b4 100644
--- a/DesignPatternsSolution/Exercise/Exercise.csproj
+++ b/DesignPatternsSolution/Exercise/Exercise.csproj
@@ -9,7 +9,7 @@
     <AppDesignerFolder>Properties</AppDesignerFolder>
     <RootNamespace>Exercise</RootNamespace>
     <AssemblyName>Exercise</AssemblyName>
-    <TargetFrameworkVersion>v4.5.2</TargetFrameworkVersion>
+    <TargetFrameworkVersion>v4.5</TargetFrameworkVersion>
     <FileAlignment>512</FileAlignment>
     <AutoGenerateBindingRedirects>true</AutoGenerateBindingRedirects>
   </PropertyGroup>
@@ -53,6 +53,7 @@
     <Compile Include="Person.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />
     <Compile Include="Registration.cs" />
+    <Compile Include="RegistrationApi.cs" />
   </ItemGroup>
   <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
   <!-- To modify your build process, add your task inside one of the targets below and uncomment it. 
diff --git a/DesignPatternsSolution/Exercise/Item.cs b/DesignPatternsSolution/Exercise/Item.cs
index 92c21e1..7000d3d 100644
--- a/DesignPatternsSolution/Exercise/Item.cs
+++ b/DesignPatternsSolution/Exercise/Item.cs
@@ -2,7 +2,8 @@
 {
     public abstract class Item : LibObject
     {
-        public Item(int amount, int year)
+        public Item(int amount, int year, IRegistrationApi registrationApi) 
+            : base(registrationApi)
         {
             AvailableAmount = amount;
             ObjType = ObjectType.Item;
@@ -12,11 +13,15 @@ public Item(int amount, int year)
 
     public class Book : Item
     {
-        public Book(string author, string title, int year, int amount) : base(amount, year)
+        public Book(string author, string title, int year, int amount, IRegistrationApi registrationApi)
+            : base(amount, year, registrationApi)
         {
             NameOrTitle = title;
             Author = author;
         }
+
         public string Author { get; set; }
+
+
     }
 }
diff --git a/DesignPatternsSolution/Exercise/LibObject.cs b/DesignPatternsSolution/Exercise/LibObject.cs
index cf1e2bb..6e0b699 100644
--- a/DesignPatternsSolution/Exercise/LibObject.cs
+++ b/DesignPatternsSolution/Exercise/LibObject.cs
@@ -1,12 +1,24 @@
 namespace Exercise
 {
-    public abstract class LibObject
+    public abstract class LibObject : IRegistarable
     {
+        private IRegistrationApi registrationApi;
+
+        protected LibObject(IRegistrationApi registrationApi)
+        {
+            this.registrationApi = registrationApi;
+        }
+
         public int ObjectId { get; set; }
         public int AvailableAmount { get; set; }
         public string NameOrTitle { get; set; }
         public ObjectType ObjType { get; set; }
         public int YearCreated { get; set; }
+
+        public RegisteredObject GetRegistrationInfo()
+        {
+            return registrationApi.GetRegistrationInfo(this);
+        }
     }
 
     public enum ObjectType
diff --git a/DesignPatternsSolution/Exercise/Library.cs b/DesignPatternsSolution/Exercise/Library.cs
index 1d9dde4..a56dcdd 100644
--- a/DesignPatternsSolution/Exercise/Library.cs
+++ b/DesignPatternsSolution/Exercise/Library.cs
@@ -3,16 +3,21 @@
     public class Library
     {
         //implement Singleton to make sure only one library will exist
+        private static Library _instance = new Library();
 
-        
+        private Library()
+        {
+        }
 
-        //Implement Register method by utilizing RegistrationRepository (complete missing parts)
-        public int Register()
+        public static Library GetInstance()
         {
-            // return RegistrationRepository.Register();
+            return _instance;
+        }        
 
-            //dummy, just to compile
-            return 0;
+        //Implement Register method by utilizing RegistrationRepository (complete missing parts)
+        public int Register(IRegistarable registrable)
+        {
+            return RegistrationRepository.Register(registrable);
         }
     }
 }
diff --git a/DesignPatternsSolution/Exercise/Person.cs b/DesignPatternsSolution/Exercise/Person.cs
index 2c2ed21..ebb1d72 100644
--- a/DesignPatternsSolution/Exercise/Person.cs
+++ b/DesignPatternsSolution/Exercise/Person.cs
@@ -7,7 +7,8 @@ public class Customer : LibObject
         public string Address { get; set; }
         public DateTime RegisteredAt { get; set; }
 
-        public Customer(string name, string addr)
+        public Customer(string name, string addr, IRegistrationApi registrationApi)
+            : base(registrationApi)
         {
             NameOrTitle = name;
             Address = addr;
diff --git a/DesignPatternsSolution/Exercise/RegistrationApi.cs b/DesignPatternsSolution/Exercise/RegistrationApi.cs
new file mode 100644
index 0000000..b931f39
--- /dev/null
+++ b/DesignPatternsSolution/Exercise/RegistrationApi.cs
@@ -0,0 +1,30 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Exercise
+{
+    public interface IRegistrationApi
+    {
+        RegisteredObject GetRegistrationInfo(LibObject libObject);
+    }
+
+    public class RegistrationApi : IRegistrationApi
+    {
+        public RegistrationApi()
+        {
+        }
+
+        public RegisteredObject GetRegistrationInfo(LibObject libObject)
+        {
+            return new RegisteredObject
+            {
+                Info = libObject.NameOrTitle,
+                AvailableAmount = libObject.AvailableAmount,
+                Id = libObject.ObjectId
+            };
+        }
+    }
+}