title: Error in dividing the columns and adding the result into a new column
body: <p>I have a dataframe 'keep_df' as shown:</p>
  
  <pre><code>            DateTime             Modeled Flow(cfs)  Observed Flow(cfs) Seconds
     Event                
     Event 1  2016-08-15 15:35:00      11.85926               0.0         300.00
     Event 1  2016-08-15 10:05:00      30.05923               0.0         300.00
     Event 1  2016-08-15 10:00:00      31.10118               0.0         300.00
     Event 1  2016-08-15 09:55:00      32.17444               0.0         300.00
     Event 1  2016-08-15 09:50:00      33.25405               0.0         300.00
  </code></pre>
  
  <p>I wanted to create new columns which is got by dividing Modeled Flow (cfs) and also Observed flow (cfs) with Seconds column as shown below:</p>
  
  <pre><code>keep_df['Modelled Volume(f3)'] = keep_df['Modeled Flow(cfs)']/keep_df['Seconds']
  keep_df['Observed Volume(f3)'] = keep_df['Observed Flow(cfs)']/keep_df['Seconds']
  keep_df()
  </code></pre>
  
  <p>But after running the above i'm getting an error like this:</p>
  
  <pre><code>Type error: 'Dataframe' object is not callable
  </code></pre>
  
  <p>Since 'Seconds' column is default value i tried this:</p>
  
  <pre><code>keep_df['Modelled Volume(f3)'] = 'Modeled Flow (cfs)'/300.00
  keep_df['Observed Volume(f3)'] = 'Observed Flow (cfs)'/300.00
  keep_df()
  </code></pre>
  
  <p>But still i'm getting this error:</p>
  
  <pre><code>TypeError                                 Traceback (most recent call last)
  &lt;ipython-input-91-b268c3093f01&gt; in &lt;module&gt;()
   ----&gt; 1 keep_df['Modelled Volume(f3)'] = 'Modeled Flow (cfs)'/300.00
    2 keep_df['Observed Volume(f3)'] = 'Observed Flow (cfs)'/300.00
    3 keep_df()
  
  TypeError: unsupported operand type(s) for /: 'str' and 'float'
  </code></pre>
  
  <p>What could i possibly do?</p>