<style>
#yellow{ width:400px; border:1px solid #FF9900; background:#FFCC99; float:left;}
#blue{ width:300px; height:100px; border:1px solid #0066FF; background:#00CCFF; float:left;}
#red{ width:100px; height:100px; border:1px solid #FF3300; background:#FF9900; float:right;}
</style>
400px
<div id="yellow">
<div id="blue">300px</div>
<div id="red">100px</div>
</div>
