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%>
<p><label for="book_title">Title</label>:
<%= radio_button 'mobile', 'brand', 'yes' :checked => checked?(
'mobile', 'brand', 'yes' )%><%= radio_button 'mobile', 'brand', no'
:checked => checked?( 'mobile', 'brand', 'no' )%></p>

<%= submit_tag "Save changes" %>
<%end %>