JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/css/bootstrap.min.css">
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/css/bootstrap-theme.min.css">
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/js/bootstrap.min.js"></script>
<script src="https://code.jquery.com/jquery-1.12.3.js"></script>
<div id="main">
  <div class="boxes">box1</div>
  <div class="boxes">box2</div>
  <div class="boxes">box3</div>
  <div class="boxes">box4</div>
  <div class="boxes">box5</div>
  <div class="boxes">box6</div>
</div>

CSS

#main {
  text-align: center;
}
.boxes {
  font-size: 30px;
  height: 200px;
  width: 45%;
  background-color: grey;
  margin: 10px;
  float: left;
}
.boxes:nth-child(4) {
    float: right;
    margin-right: calc(100% - (90% + 30px)) !important;
}
.boxes:nth-child(6n-2) {
  font-size: 30px;
  height: 420px;
  margin: 10px;
}
.boxes:nth-child(6n-5) {
  font-size: 30px;
  height: 420px;
  margin: 10px;
}