diff --git a/app/models/exercise.rb b/app/models/exercise.rb
index 152d79bf..b6ff3bd1 100644
--- a/app/models/exercise.rb
+++ b/app/models/exercise.rb
@@ -23,14 +23,19 @@ class Exercise < ActiveRecord::Base
   validates :token, presence: true, uniqueness: true
 
   def average_percentage
-    (average_score/ maximum_score * 100).round if average_score
+    (average_score / maximum_score * 100).round if average_score
   end
 
   def average_score
     if submissions.exists?(cause: 'submit')
       maximum_scores_query = submissions.select('MAX(score) AS maximum_score').where(cause: 'submit').group(:user_id).to_sql.sub('$1', id.to_s)
       self.class.connection.execute("SELECT AVG(maximum_score) AS average_score FROM (#{maximum_scores_query}) AS maximum_scores").first['average_score'].to_f
-    end
+    else 0 end
+  end
+
+  def average_working_time
+    working_time_query = submissions.select('MAX(created_at) - MIN(created_at) AS time').group(:user_id).to_sql.sub('$1', id.to_s)
+    self.class.connection.execute("SELECT AVG(time) AS average_time FROM (#{working_time_query}) AS working_times").first['average_time']
   end
 
   def duplicate(attributes = {})
diff --git a/app/views/execution_environments/statistics.html.slim b/app/views/execution_environments/statistics.html.slim
index 111c6d29..38112c4f 100644
--- a/app/views/execution_environments/statistics.html.slim
+++ b/app/views/execution_environments/statistics.html.slim
@@ -9,15 +9,7 @@ h1 = @execution_environment
     tbody
       - @execution_environment.exercises.each do |exercise|
         tr
-          - submissions = exercise.submissions
           td = exercise.title
-          td = submissions.average(:score)
-          td = submissions.count()
-          - minima = submissions.group(:user_id).minimum(:created_at)
-          - maxima = submissions.group(:user_id).maximum(:created_at)
-          - result = 0
-          - results = {}
-          - maxima.each {|key, value| results[key] = value - minima[key]}
-          - results.values.map {|value| result += value}
-          - result /= results.size if results.size > 0
-          td = distance_of_time_in_words(result)
+          td = exercise.average_score
+          td = exercise.submissions.count()
+          td = exercise.average_working_time