边距问题，为什么一个是width:770，另一个却要是width:730才能合适？
<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>无标题文档</title>
<style type="text/css">
*{margin:0;padding:0;}
#wrap{width:770px;margin:0 auto;}
#top{width:100%;height:100px;border:1px solid red;}
#mybody{width:100%;height:500px;border:1px solid blue;}
#foot{width:100%;height:200px;border:1px solid green;}
img{margin:10px 18px;border:1px solid #999;}
.content{border:4px solid #def4f5;padding:74px 15px;width:770px;}
.book{background:url(../%E7%BD%91%E9%A1%B5%E5%B8%83%E5%B1%80%E5%9F%BA%E7%A1%80/case2/images/t_book.gif) no-repeat #def4f5;}
</style>
</head>
<body>
<div id="wrap">
<div id="top"></div>
<div id="mybody">
<div class="content book">
<img src="../网页布局基础/case2/images/book1.jpg" width="80" height="80" alt=""/>
<img src="../网页布局基础/case2/images/book2.jpg" width="80" height="80" alt=""/>
<img src="../网页布局基础/case2/images/book3.jpg" width="80" height="80" alt=""/>
<img src="../网页布局基础/case2/images/book4.jpg" width="80" height="80" alt=""/>
<img src="../网页布局基础/case2/images/book5.jpg" width="80" height="80" alt=""/>
</div>
</div>
<div id="foot"></div>
</div>
</body>
</html>
