title: Selecting the right nth-childs
body: <p>easy question, since my math skills fails. I need to target the following numbers of elements by css in a container by a nth-child calculation:</p>
  
  <p>6, 9, 21, 24, 36, 39, 51, 54, 66, 69 and so on</p>
  
  <p>Starting at 6, 9, 21, 24 there are always gaps of 30 elements (6 -> 36 / 9 -> 39 / 21 -> 51/ 24 -> 54 / 36 -> 66)</p>
  
  <p>Any way to target those elements via css (maybe one ore few rules of nth-childs?), since it is an infinite loop of article? So the number could increase in the future, but the grid system will stay the same.</p>