Мое решение. Не знаю правильно ли, ну как понял.
<!DOCTYPE html>
<html lang="ru">
<head>
<title>Испытание: строим сетку</title>
<meta charset="utf-8">
<link rel="stylesheet" href="style.css">
</head>
<body>
<div class="raw clearfix">
<div class="header layout-column">Header</div>
<div class="menu">
<div class="centr-menu layout-column">
Menu
</div>
</div>
<div class="promo layout-column">Promo 1</div>
<div class="promo layout-column">Promo 2</div>
</div>
<div class="main clearfix">
<div class="left layout-column">Left</div>
<div class="centr layout-column">Main</div>
<div class="layout-column">Right</div>
</div>
<div class="footer clearfix">
<div class="layout-column">Footer</div>
</div>
</body>
</html>
body {
width: 450px;
height: 335px;
font-size: 10px;
font-family: "Arial", sans-serif;
color: white;
}
.raw {
background-color: #34495e;
min-height: 170px;
}
.header {
background-color:#c0392b;
float: left;
width:350px;
min-height:35px;
margin: 10px 50px;
}
.menu {
background-color:#3498db;
clear:both;
min-height: 35px;
}
.centr-menu {
width:350px;
min-height: inherit;
margin: auto;
}
.promo {
background-color:#c0392b;
float:left;
width:170px;
min-height: 60px;
margin: 10px;
margin-left: 50px;
}
.promo:last-child {
float:right;
margin-right: 50px;
margin-left: 0px;
}
.man {
min-height: 130px;
}
.main .layout-column{
background-color:#3498db;
float:left;
min-height: 110px;
width: 70px;
margin: 10px 5px;
}
.main .left{
margin-left: 50px;
}
.main .centr{
width: 190px;
}
.footer {
background-color: #34495e;
min-height: 35px;
}
.footer .layout-column {
float: left;
width: 350px;
height: 35px;
margin: 0 50px;
}
.layout-column{
padding: 5px;
box-sizing:border-box;
}
.clearfix::after {
content: "";
display: table;
clear: both;
}