diff --git a/udemy-course/src/challenges/library-challenge.kt b/udemy-course/src/challenges/library-challenge.kt
index 3bef754..5c23c6d 100644
--- a/udemy-course/src/challenges/library-challenge.kt
+++ b/udemy-course/src/challenges/library-challenge.kt
@@ -1,74 +1,78 @@
 package challenges
 
-/**
- * @author Peter Sommerhoff
- *
- * Solution for the coding challenge on object orientation inside the Udemy course.
- * Defines some simple interfaces and classes for a library inventory system.
- */
-interface Lendable {
+interface Lendable
+{
     fun borrow()
+    fun returnItem()
 }
 
-// The properties title, genre, and publicationYear can be included in the parent class because both books and DVDs
-// have these properties. The author and length properties however are included only in the child classes.
-abstract class InventoryItem(val title: String,
-                             val genre: String,
-                             var publicationYear: Int,
-                             var borrowed: Boolean) : Lendable {
+interface Viewable
+{
+    fun view()
+}
+
+abstract class InventoryItem(
+    val title: String,
+    val genre: String,
+    val publicationYear: Int
+): Lendable, Viewable
+{
+    private var borrowed = false
+    private var returned = true
 
     override fun borrow() {
-        if (!borrowed) {
-            borrowed = true
-        } else {
-            println("This item is already borrowed.")
-        }
+        this.borrowed = true
     }
 
-    // If you want, you can also add a returnItem() method so that you can also un-borrow items.
-
+    override fun returnItem() {
+        this.returned = true
+    }
 
     override fun toString(): String {
-        return "InventoryItem(title='$title', genre='$genre', publicationYear=$publicationYear, borrowed=$borrowed)"
+        return "InventoryItem(title=$title, genre=$genre, year=$publicationYear, borrowed=$borrowed"
     }
-
-    abstract fun copy(): InventoryItem
-
 }
 
-// A book will be not borrowed by default.
-// The class is called LibraryBook to prevent name clashes with previous challenges.
-class LibraryBook(title: String,
-                  val author: String,
-                  genre: String,
-                  publicationYear: Int) : InventoryItem(title, genre, publicationYear, false) {
-
-    fun read() {
-        println("Reading a book by $author...")
+class Book(
+    title: String,
+    genre: String,
+    publicationYear: Int,
+    private val author: String
+): InventoryItem(title, genre, publicationYear)
+{
+    override fun view() {
+        println("Reading $title ($genre) by $author published $publicationYear ...")
     }
+}
 
-    override fun copy(): InventoryItem {
-        val copy = LibraryBook(title, author, genre, publicationYear)
-        copy.borrowed = this.borrowed
-
-        return copy
+class Dvd(
+    title: String,
+    genre: String,
+    publicationYear: Int,
+    private val lengthInMinutes: Int
+): InventoryItem(title, genre, publicationYear)
+{
+    override fun view() {
+        println("Watching $title ($genre). Length $lengthInMinutes min. Released $publicationYear")
     }
 }
 
-// A DVD will also be not borrowed by default.
-class LibraryDVD(title: String,
-                 genre: String,
-                 val length: Int,
-                 publicationYear: Int) : InventoryItem(title, genre, publicationYear, false) {
+fun main() {
+    val book = Book(
+        "A Book Title",
+        "A Fun Genre",
+        2000,
+        "An Author Name"
+    )
 
-    fun watch() {
-        println("Watching $title...")
-    }
+    println(book)
 
-    override fun copy(): InventoryItem {
-        val copy = LibraryDVD(title, genre, length, publicationYear)
-        copy.borrowed = this.borrowed
+    val dvd = Dvd(
+        "A DVD Title",
+        "A Fun Genre",
+        2000,
+        180
+    )
 
-        return copy
-    }
-}
\ No newline at end of file
+    println(dvd)
+}