title: Excel Auto number row based on criteria
body: <p>I'm trying to create a spreadsheet in excel which creates a sequential number in a column (B) depending on the contents of another column. Currently, there are two possibilities of what could be in Column A ("BI" or "GF"). So I want the data to look like this</p>
  
  <pre><code>    COL A    COLB
       BI       1
       BI       2
       GF       1
       BI       3
       GF       2
       GF       3
       BI       4
       BI       5
  </code></pre>
  
  <p>I've tried several attempts to do this but can't seem to find a solution. Any help would be greatly appreciated. </p>