Drupal 7 Form API - Reveal Input on drop down condition

Here’ the task :  I have a drop down list , a simple ‘yes’ or ‘no’  .  If yes fill in the the input box. 

At first I thought this would need a simple Ajax form call but in fact its even easier than that.   We can use the ‘States’ and ‘visible’ values of the Form API [https://api.drupal.org/api/examples/form_example%21form_example_states.inc/function/form_example_states_form/7]

Here’s my code that solved this issue

$form['addtoportfolio'] = array(
  '#type' => 'select',
  '#prefix' => "Your score was $score%; the pass rate is $pass_rate% ",
  '#title' => t('Would you like to add this to your portfolio ?'),
  '#required' => TRUE,
  '#options' => array(
    'n' => t('No'),
    'y' => t('Yes'),
  ),
  '#default_value' => t('No'),
  );



$form['timetaken'] = array(
  '#type' => 'textfield',
  '#title' => t('Time Taken ?'),
  '#required' => FALSE,
  '#description' => "Please enter the time in minutes that it took you to do this quiz.",
  '#size' => 4,
  '#maxlength' => 10,
  '#states' => array(
    'visible' => array(
      ':input[name="addtoportfolio"]' => array('value' => 'y'),
    ),
  ),

);