Fix rubocop offenses

app/models/exercise.rb

@@ -68,7 +68,9 @@ class Exercise < ApplicationRecord
     if submissions.exists?(cause: 'submit')
       maximum_scores_query = submissions.select('MAX(score) AS maximum_score').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
-    else 0 end
+    else
+      0
+    end
   end
 
   def average_number_of_submissions

app/models/proxy_exercise.rb

@@ -240,7 +240,7 @@ class ProxyExercise < ApplicationRecord
 
   def tag_diminishing_return_function(count_tag, total_count_tag)
     total_count_tag += 1 # bonus exercise comes on top
-    1 / (1 + (Math::E**(-3 / (0.5 * total_count_tag) * (count_tag - 0.5 * total_count_tag))))
+    1 / (1 + (Math::E**(-3 / (0.5 * total_count_tag) * (count_tag - (0.5 * total_count_tag)))))
   end
 
   def select_easiest_exercise(exercises)