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

$width: 50px;
$height: 50px;

* {
  margin: 0;
  padding: 0;
}


.gray {
  width: 100%;
  height: 700px;
  background-color: gray;
}

.blue {
  width: $width;
  height: $height;
  background-color: blue;
  position: absolute;
}