Edit radio button values

my problem is… during edit process radio button should be checked as
default …how can i do that …pl help…

my edit .erb file

<% form_tag (:action => ‘update’, :id => @mobile )do%>

Title: <%= radio_button 'mobile', 'brand', 'yes' :checked => checked?( 'mobile', 'brand', 'yes' )%><%= radio_button 'mobile', 'brand', no' :checked => checked?( 'mobile', 'brand', 'no' )%>

<%= submit_tag “Save changes” %>
<%end %>