extract details_known_question_id

app/models/form/sales/pages/person.rb

@@ -11,4 +11,8 @@ class Form::Sales::Pages::Person < ::Form::Page
   def joint_purchase?
     id.include?("_joint_purchase")
   end
+
+  def details_known_question_id
+    "details_known_#{person_display_number}"
+  end
 end

app/models/form/sales/pages/person_age.rb

@@ -5,7 +5,7 @@ class Form::Sales::Pages::PersonAge < Form::Sales::Pages::Person
     @description = ""
     @subsection = subsection
     @depends_on = [
-      { "details_known_#{person_display_number}" => 1, "jointpur" => joint_purchase? ? 1 : 2 },
+      { details_known_question_id => 1, "jointpur" => joint_purchase? ? 1 : 2 },
     ]
   end
 

app/models/form/sales/pages/person_working_situation.rb

@@ -5,7 +5,7 @@ class Form::Sales::Pages::PersonWorkingSituation < Form::Sales::Pages::Person
     @description = ""
     @subsection = subsection
     @depends_on = [
-      { "details_known_#{person_display_number}" => 1, "jointpur" => joint_purchase? ? 1 : 2 },
+      { details_known_question_id => 1, "jointpur" => joint_purchase? ? 1 : 2 },
     ]
   end