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function adjustIframes() {
  $('iframe').each(function(){
    var $this = $(this),
    proportion = $this.data('proportion'),
    w = $this.attr('width'),
    actual_w = $this.width();

    if (! proportion) {
        proportion = $this.attr('height') / w;
        $this.data( 'proportion', proportion );
    }

    if (actual_w != w) {
        $this.css( 'height', Math.round( actual_w * proportion ) + 'px' );
    }
  });
}
$(window).on('resize load', adjustIframes);

<!-- All you need is a defined width & height -->
<iframe frameborder="0" height="426" src="http://www.slideshare.net/slideshow/embed_code/9812085?rel=0" width="510"></iframe>

iframe {
  max-width: 100%;
}