title: Multiplying a Pandas column by a multi-index value
body: <p>I had a multi-index dataframe with three levels on index. I would like to multiply each Percent value by the total population.</p>
  
  <p>I am trying something like this:</p>
  
  <pre><code>df = df.reset_index(level=2, drop=True)
  df['TOTAL'] = df['POP.'] * output['PERCENT']
  </code></pre>
  
  <p>I get <code>KeyError: 'POP.'</code></p>
  
  <p>Starting DataFrame</p>
  
  <pre><code>                               PERCENT  
       DATE      POP.  SEX                
  2015-01-01     100  MALE        0.51  
                      FEMALE      0.49  
  2016-01-01     200  MALE        0.52  
                      FEMALE      0.48
  </code></pre>
  
  <p>Desired Output</p>
  
  <pre><code>                               PERCENT     TOTAL  
       DATE      POP.  SEX                
  2015-01-01     100  MALE        0.51         51  
                      FEMALE      0.49         49
  2016-01-01     200  MALE        0.52        104
                      FEMALE      0.48         96
  </code></pre>