title: Check if margin-left is a certain percentage
body: <p>I ran into a problem when making an easy horizontal slider.
  With a right and left arrow (img) the position goes -100% or +100% and this works fine! :)
  When the slider is at -400%, the right arrow should disappear, but when i use jquery to check this, nothing happens? Also when the left arrow is at 0% it should disappear.</p>
  
  <p>Here is my code:</p>
  
  <pre><code>$('.right img').live('click', function(){
      $('.play').stop(true, true).animate({
          marginLeft: "-=100%"
      }, 1000);
  $('.left img').css("visibility", "visible")
  if($('.play').css("margin-left")="400%") {
      $('.right img').css("visibility", "hidden")     
      }
  });
  
  $('.left img').live('click', function(){
      $('.play').stop(true, true).animate({
          marginLeft: "+=100%"
      }, 1000);
  
  if($('.play').css('margin-left')=='-2520px'){           
      $('.right img').css("visibility", "visible")
  }
  else if($('.play').css("margin-left")=0) {
      $('.left img').css("visibility", "hidden")
  }
  });
  </code></pre>
  
  <p>When i use px instead of % it works! But for a responsive code i would like to use %.</p>
  
  <p>Sorry for my bad english :)
  Thanks for your time!!</p>