<div id="container">
    <div id="side"> panel A</div>
    <div id="head"> panel B</div>
    <div id="content"> panel C</div>
</div>

#container{
    width: 100%;
}
#side{
    width: 20%;
    float: left;
}
#head{
    width: 80%;
    float: left;
}
#content{
    width: 80%;
    float: left;
}

<div id="container">
    <div id="side"> panel A</div>
    <div class="wrapper">
        <div id="head"> panel B</div>
        <div id="content"> panel C</div>
    </div>
</div>

#container{
    width: 80%;
    padding: 0 0 0 20%;
}
#side{
    width: 20%;
    float: left;
    margin: 0 0 0 -20%;
}