title: What can this small Excel formula do?
body: <p>TDay = Daytime Temperatures. Column A contains names of the months.</p>
  
  <pre><code>=AVERAGE(INDEX(TDay; 1+($A2-1)*12):INDEX(TDay; A2*$12))
  </code></pre>
  
  <p>The formula doesn't work, some error with row_num.</p>