<ul id="box">
<li>a</li> <li>b</li> <li>c</li> </ul>
#box{display:box;width:240px;height:100px;margin:0;padding:10px;list-style:none;}
#box li:nth-child(1){box-flex:1;} #box li:nth-child(2){box-flex:1;} #box li:nth-child(3){box-flex:2;}
<ul id="box">
<li>a</li> <li>b</li> <li>c</li> </ul>
#box{display:box;width:240px;height:100px;margin:0;padding:10px;list-style:none;}
#box li:nth-child(1){box-flex:1;} #box li:nth-child(2){box-flex:1;} #box li:nth-child(3){box-flex:2;}