title: average of two columns for each row in pandas
body: <p>I am a bit new in pandas and i have a questions
  I want to divide two columns by row.</p>
  <p>More about the data: I have several companies, who sold n items for m revenue</p>
  <pre><code>company #   nr of items revenue 
  1               30      32.0          
  2               40      59.3     
  3               22.0    64  
  </code></pre>
  <p>I want to calculate</p>
  <pre><code>    company #   nr of items revenue average price per item
  1               30           32.0          
  2               40           59.3     
  3               22.0         64  
  </code></pre>
  <p>average price per item = revenue /nr of items</p>
  <p>I tried</p>
  <pre><code>df[['company','revenue','nr of items']].groupby('company')['revenue'].sum()/['nr of items'].sum() 
  </code></pre>
  <p>but it doesn't work...
  Any ideas?
  Thanks!</p>