Bootstrap 3.0 - Grid System

HTML

<link rel="stylesheet" href="//netdna.bootstrapcdn.com/bootstrap/3.0.0/css/bootstrap.min.css">
<script src="//netdna.bootstrapcdn.com/bootstrap/3.0.0/js/bootstrap.min.js"></script>
<link rel="stylesheet" href="//netdna.bootstrapcdn.com/font-awesome/3.2.1/css/font-awesome.min.css">
<div class='row'>
    <div class="col-md-6">
      <div class='row'>
      <div class="col-md-6">
        <div class='row'>
          <div class="col-md-1">1+</div>
          <div class="col-md-6">6+</div>
          <div class="col-md-5">5=12</div>
        </div>
      </div>
            <div class="col-md-6">
        <div class='row'>
          <div class="col-md-2">2+</div>
          <div class="col-md-6">6+</div>
          <div class="col-md-4">4=12</div>
        </div>
      </div>
      </div>
    </div>
    <div class="col-md-6">test</div>
</div>

CSS

.row > div {
    border: 1px solid grey;
}