Add link to original problem

app/models/problem.rb

@@ -3,4 +3,8 @@ class Problem < ActiveRecord::Base
   validates :title, uniqueness: true
 
   self.per_page = 50
+
+  def original_url
+    "https://projecteuler.net/problem=#{id}"
+  end
 end

app/views/problems/show.html.erb

@@ -11,5 +11,7 @@
         <%= sanitize @problem.content %>
     </div>
 <% end %>
-
+<div class="text-center">
+  <%= link_to 'Dieses Problem auf projecteuler.net', @problem.original_url, target: '_blank' %>
+</div>
 <%= link_to 'Edit', edit_problem_path(@problem), class: 'btn btn-default' %>