Edit in JSFiddle

<h1>100% ширина контейнера</h1>

<div class="container">
  <section>Section</section>
  <aside>Aside</aside>
</div>

<h1>75% ширина контейнера</h1>

<div class="container container-75">
  <section>Section</section>
  <aside>Aside</aside>
</div>

<h1>50% ширина контейнера</h1>

<div class="container container-50">
  <section>Section</section>
  <aside>Aside</aside>
</div>

body {
  font: 600 14px/24px "Open Sans", "HelveticaNeue-Light", "Helvetica Neue Light", "Helvetica Neue", Helvetica, Arial, "Lucida Grande", Sans-Serif;
}
h1 {
  color: #9799a7;
  font-size: 14px;
  font-weight: bold;
  margin-bottom: 6px;
}
.container:before,
.container:after {
  content: "";
  display: table;
}
.container:after {
  clear: both;
}
.container {
  background: #eaeaed;
  margin-bottom: 24px;
  *zoom: 1;
}
.container-75 {
  width: 75%;
}
.container-50 {
  margin-bottom: 0;
  width: 50%;
}
.container,
section,
aside {
  border-radius: 6px;
}
section,
aside {
  background: #2db34a;
  color: #fff;
  margin: 1.858736059%;
  padding: 20px 0;
  text-align: center;
}
section {
  float: left;
  width: 63.197026%;
}
aside {
  float: right;
  width: 29.3680297%;
}