title: SASS/Compass Math using dynamic width
body: <p>I have a container div of flexible width (90%). I would like its child div to always be 180px smaller than the parent, in terms of width.</p>
  
  <p>I know this can be accomplished in JS but I'm trying to use as much CSS as possible. I have a feeling this is beyond what CSS can do, but wanted to be sure.</p>
  
  <p>I want to get the width of the child (in px) and subtract 180px. Is that possible?</p>
  
  <p><strong>Markup:</strong></p>
  
  <pre><code>&lt;div id="main"&gt;
      &lt;div class="block"&gt;&lt;/div&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p><strong>SCSS:</strong></p>
  
  <pre><code>#main {
    width: 90%;
    max-width: 1600px;
  
    .block {
      /* incorrect code: */
      width: WIDTH_OF_PARENT - 180px;
    }
  }
  </code></pre>