Edit in JSFiddle

<div class="flex-container row bg">
<div class="flex-item">1</div>
<div class="flex-item">2</div>
<div class="flex-item">3</div>
<div class="flex-item">4</div>
<div class="flex-item">5</div>
</div>

<div class="flex-container row bg">
<div class="flex-item">1</div>
<div class="flex-item two">2</div>
<div class="flex-item">3</div>
<div class="flex-item">4</div>
<div class="flex-item">5</div>
</div>

.flex-container {
  padding: 0;
  margin: 0;
  list-style: none;
  
  -ms-box-orient: horizontal;
  display: -webkit-box;
  display: -moz-box;
  display: -ms-flexbox;
  display: -moz-flex;
  display: -webkit-flex;
}
.row           { 
  -webkit-flex-direction: row; 
  flex-direction: row;
  flex-wrap: normal;
}

.flex-item {
  background: tomato;
  padding: 5px;
  width: 50px;
  height: 50px;
  margin: 5px;
  
  line-height: 50px;
  color: white;
  font-weight: bold;
  font-size: 2em;
  text-align: center;
  flex-grow: 1;
}

.two {
	background: gold;
	flex-grow: 3;
}

.bg {
	background: lightgray;
}