title: Arithmetic operation on a groupby pandas dataframe
body: <p>I have a pandas dataframe with 40 columns and 400000 rows. I created a rolled up dataset on 3 columns.</p>
  
  <p>Now, I need to compute a % metric based on two of the columns. Python throws an error -   </p>
  
  <pre><code>unsupported operand type(s) for /: 'SeriesGroupBy' and 'SeriesGroupBy'
  </code></pre>
  
  <p>here's the sample code : </p>
  
  <pre><code>print sample_data
     date  part  receipt  bad_dollars  total_dollars  bad_percent
  0     1   123       22           40            100          NaN
  1     2   456       44           80            120          NaN
  2     3   134       33           30            150          NaN
  3     1   123       22           80            100          NaN
  4     5   456       45           40             90          NaN
  5     3   134       33           85            150          NaN
  6     7   123       24           70            120          NaN
  7     5   456       45           20             85          NaN
  8     9   134       35           50            300          NaN
  9     7   123       24          300            600          NaN
  
  sample_data_group = sample_data.groupby(['date','part','receipt'])
  
  sample_data_group['bad_percents']=sample_data_group['bad_dollars']/sample_data_group['total_dollars']
  
  TypeError: unsupported operand type(s) for /: 'SeriesGroupBy' and 'SeriesGroupBy'
  </code></pre>
  
  <p>Please help!</p>