html,
body { height: 100%; padding: 0; margin: 0; }
.outer { height: 100%; position: relative; }
.A { height: 100px; background: #BBE8F2; }
.B { background: #D9C666; width: 100%; position: absolute; top: 100px ; left: 0 ; bottom: 0; }
</style>
<div class="outer">
<div class="A"></div>
<div class="B">
<div class="leftp" style="width:300px;height:100%;background-color:red;float:left;">
<div class="rightp" style="margin-left:300px;height:100%">
</div>
</div>