A CodePen by Hugo Giraudel. Demo Flexbox 1

index.html

<ul class="flex-container">
  <li class="flex-item item1">1</li>
  <li class="flex-item item2">2</li>
  <li class="flex-item item3">3</li>
  <li class="flex-item item4">4</li>
  <li class="flex-item item5">5</li>
  <li class="flex-item item6">6</li>
</ul>

style.scss

.flex-container {
  padding: 10px;
  list-style: none;
  
  display: -webkit-box;
  display: -moz-box;
  display: -ms-flexbox;
  display: -webkit-flex;
  display: flex;
  
  -webkit-flex-flow: row wrap;
  justify-content: space-between;
}

.flex-item {
  background: tomato;
  padding: 5px;
  height: 150px;
  margin-top: 10px;
  
  line-height: 150px;
  color: white;
  font-weight: bold;
  font-size: 3em;
  text-align: center;
  &.item1{
    width: 25%;
  }
  &.item2{
    width:10%;
  }
  &.item3{
    width:10%;
  }
  &.item4{
    width: 10%;
  //  flex-grow:;
  }
  &.item5{
    width: 10%;
  }
  &.item6{
    width: 20%;
  }
}