CodePen Embed - OQbYyb

<div class="box"></div>
<div class="box230px"></div>

body {
  width: 200px;
}

.box {
  width: 100%;
  height: 100px;
  background: #CCC;
}

.box230px {
  width: calc(100% + 30px);
  height: 100px;
  background: #000;
}

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