Fix number of runs

app/models/exercise.rb

@@ -37,6 +37,11 @@ class Exercise < ActiveRecord::Base
     else 0 end
   end
 
+  def average_number_of_submissions
+    user_count = internal_users.distinct.count + external_users.distinct.count
+    return user_count == 0 ? 0 : submissions.count() / user_count.to_f()
+  end
+
   def average_working_time
     self.class.connection.execute("""
       SELECT avg(working_time) as average_time

app/views/execution_environments/statistics.html.slim

@@ -11,5 +11,5 @@ h1 = @execution_environment
         tr
           td = exercise.title
           td = exercise.average_score
-          td = exercise.submissions.count()
+          td = exercise.average_number_of_submissions
           td = exercise.average_working_time