title: What's an efficient way to select every 6th and 7th element using css nth-child
body: <p>I'm using the css nth-child selector to try to add a 20px margin to every 6th AND 7th element in an unordered list.</p>
  
  <pre><code>1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
            X X        X  X           X  X           X  X
  </code></pre>
  
  <p>So far I've accomplished this by using two separate css selectors:</p>
  
  <pre><code>li:nth-child(5n+6){}
  li:nth-child(5n+7){}
  </code></pre>
  
  <p>Is there a way to accomplish this pattern using only one nth-child formula?</p>