diff --git a/app/models/issue.rb b/app/models/issue.rb
index ab9f794db..f0616c138 100644
--- a/app/models/issue.rb
+++ b/app/models/issue.rb
@@ -1838,19 +1838,20 @@ class Issue < ActiveRecord::Base
           if children.any?
             child_with_total_estimated_hours = children.select {|c| c.total_estimated_hours.to_f > 0.0}
             if child_with_total_estimated_hours.any?
-              average =
-                child_with_total_estimated_hours.sum(&:total_estimated_hours).to_d /
-                  child_with_total_estimated_hours.count
+              average = Rational(
+                child_with_total_estimated_hours.sum(&:total_estimated_hours).to_s,
+                child_with_total_estimated_hours.count
+              )
             else
-              average = BigDecimal('1.0')
+              average = Rational(1)
             end
             done = children.sum do |c|
-              estimated = (c.total_estimated_hours || 0.0).to_d
+              estimated = Rational(c.total_estimated_hours.to_f.to_s)
               estimated = average unless estimated > 0.0
               ratio = c.closed? ? 100 : (c.done_ratio || 0)
               estimated * ratio
             end
-            progress = done / (average * children.count)
+            progress = Rational(done, average * children.count)
             p.done_ratio = progress.floor
           end
         end
diff --git a/test/unit/issue_subtasking_test.rb b/test/unit/issue_subtasking_test.rb
index 0c2ffaa4d..0710190fd 100644
--- a/test/unit/issue_subtasking_test.rb
+++ b/test/unit/issue_subtasking_test.rb
@@ -243,11 +243,18 @@ class IssueSubtaskingTest < ActiveSupport::TestCase
 
   def test_done_ratio_of_parent_with_completed_children_should_not_be_99
     with_settings :parent_issue_done_ratio => 'derived' do
-      parent = Issue.generate!
-      parent.generate_child!(:estimated_hours => 8.0, :done_ratio => 100)
-      parent.generate_child!(:estimated_hours => 8.1, :done_ratio => 100)
+      parent1 = Issue.generate!
+      parent1.generate_child!(:estimated_hours => 8.0, :done_ratio => 100)
+      parent1.generate_child!(:estimated_hours => 8.1, :done_ratio => 100)
       # (8.0 * 100 + 8.1 * 100) / (8.0 + 8.1) => 99.99999999999999
-      assert_equal 100, parent.reload.done_ratio
+      assert_equal 100, parent1.reload.done_ratio
+
+      parent2 = Issue.generate!
+      parent2.generate_child!(:estimated_hours => 9.0, :done_ratio => 100)
+      10.times do
+        parent2.generate_child!(:estimated_hours => 10.0, :done_ratio => 100)
+      end
+      assert_equal 100, parent2.reload.done_ratio
     end
   end