*{ margin:0; padding:0; }
.main{ margin:50px auto; width:90%; outline:#000 1px solid; overflow:hidden}
.a,.b,.c{ background:#CCC; height:100px; float:left;width:50%; }
.b{ background:#999; width:200px;}
.c{ margin-right:-100px;}
.a{ margin-right:-100px;}