Как установить атрибут в процентах в jQuery?

var targets = $('.parallax__layer__cell');
var i ;
 for(i = 0; i < targets.length; i++)
{
  if (targets.eq(i).parents('div.parallax__layer--bg').length) {
          targets.eq(i).css('left', 60*i +  "%");
   }
}

.parallax__layer__cell{
position:absolute;
}

<script src = "https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div class = "wrap">
  <div class = "parallax__layer--bg">
    <div class = "parallax__layer__cell">a</div>
  </div> 

  <div class = "parallax__layer--bg">
    <div class = "parallax__layer__cell">b</div>
  </div>

  <div class = "parallax__layer--bg">
    <div class = "parallax__layer__cell">c</div>
  </div>

  <div class = "parallax__layer--bg">
    <div class = "parallax__layer__cell">d</div>
  </div>

  <div class = "parallax__layer--bg">
    <div class = "parallax__layer__cell">e</div>
  </div>

  <div>
    <div class = "parallax__layer__cell">--no parent--</div>
  </div>
</div>