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<div class="box">
  <div class="blue"></div>
  <div class="yellow"></div>
  <div class="red"></div>
</div>

* {
  margin: 0;
  padding: 0;
}

.box {
  width: 100%;
  height: 300px;
  background-color: green;
}

.blue {
  width: 50px;
  height: 100px;
  background-color: blue;
}

.yellow {
  width: 100px;
  height: 50px;
  background-color: yellow;
  float: left;
}

.red {
  width: 80px;
  height: 80px;
  background-color: red;
  float: left;
}