calc用法

		<ul>
			<li></li>
			<li></li>
			<li></li>
		</ul>


    <style type="text/css">
        html {height:100%;font-size:62.5%;}
	body {height:100%;font:normal normal 1.2em/1.8 'Microsoft YaHei',Tahoma,Arial,SimSun,sans-serif;color:#4a4439;background: #6D928B;max-width: 640px;margin: 0 auto;}
        ul,li,ol{list-style-type:none;}
	ul{width: 100%;overflow: hidden;}
        li{
            float:left;
	    width:33.3333%;
	    height:50px;
	    padding:10px;
	    margin-right:15px;
	    background:#FF6666;
	    border:5px solid #DAC8A7;
	    width:calc(33.3333% - (10px + 5px) * 2 - 15px );
	    /*意思是(width-(padding+border)*2-margin)*/
        }
    </style>