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

<p>実際の幅は200px</p>
.outer {
  width: 200px;
  background: #c0ffee;
}

.box {
  width: 160px; /* 200px - 20px - 20px */
  height: 40px;
  border: 1px solid #999;
  padding: 20px;
}

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