Siatka flex w css

<div class="container">
  <div class="row">
    <div class="col"></div>
    <div class="col">
      <div class="col"></div>
      <div class="col"></div>  
    </div>
  </div>
  <div class="row">
      <div class="col"></div>
      <div class="col"></div>  
      <div class="col"></div>
      <div class="col"></div>  
  </div>
</div>

* { box-sizing: border-box; }
.container {
  width: 80%;
  margin: 0 auto;
}
.row {
  display: flex;
  width: 100%;
}
.col {
  flex: 1;
  border: 5px solid black;
  min-height: 150px;
  margin: 5px;
}