Q25 下のような配置 

<!DOCTYPE html>
<html lang="ja">
<head>
<meta charset="UTF-8">
<title>Q25</title>
<link rel="stylesheet" href="css/style.css">
</head>
<body>
<div id="wrapper">
<div id="blocka">ブロックA</div>
<div id="blockb">ブロックB</div>
<div id="blockc">ブロックC</div>
<div id="blockd">ブロックD</div>
</div> <!-- /#wrapper -->
</body>
</html>



@charset="UTF-8";

/*reset*/
body,div {
  margin: 0;
  padding: 0;
  font-family:
    "Hiragino Kaku Gothic ProN",
    Meiryo,
    sans-serif;
}

/*style*/
#wrapper {
  width: 400px;
  margin: 10px auto;
}
#blocka {
  width: 400px;
  height: 40px;
  background: yellow;
  margin-bottom: 5px;
}
#blockb {
  width: 195px;
  height: 40px;
  margin-right: 5px;
  background: skyblue;
  float: left;
  margin-bottom: 5px;
}
#blockc {
  width: 195px;
  height: 40px;
  margin-left: 5px;
  background: lightgreen;
  float: right;
  margin-bottom: 5px;
}
#blockd {
  width: 400px;
  height: 40px;
  background: lightpink;
  clear: both;
}