$(document).ready( function() {
$('.parent-div .div:last').addClass('noMargin')
});
<div class="parent-div"> <div class="div">foo</div> <div class="div">foo</div> <div class="div">foo</div> <div class="div">foo</div> <div class="div">foo</div> </div>
.div{width:100px;height:100;float:left;background:#000;margin-left:10px;color:#fff;}
.noMargin{margin:0;}