title: SASS Math Calculation
body: <p>Trying to create a responsive grid of videos. Instead of using different %'s for each media query, I was hoping to use a standard SASS formula that calculates based on 100% width, however not sure if SASS can do this. The 40 in the below formula, takes into account 2 x 20px fixed margins (i.e this would be a 3-column grid).</p>
  
  <p><strong>Ideal formula:</strong></p>
  
  <pre><code>ul.videos {
    li {
       width: ((100% / 3) - 40);
    }
  }
  </code></pre>
  
  <p>Any way that CSS/SASS can handle this? Would prefer not to use JS if possible. </p>