title: Multiply two columns in a groupby statement in pandas
body: <p>I have a simplified data frame called df</p>
  
  <pre><code>import pandas as pd
  df = pd.DataFrame({'num': [1,1,2,2],
                 'price': [12,11,15,13],
                 'y': [7,7,9,9]})
  </code></pre>
  
  <p>I want to group by num an then multiply price and y and take the sum divided by sum of y</p>
  
  <p>I've been trying to get started with this and have been having trouble</p>
  
  <pre><code>df.groupby('letter').agg(['price']*['quantity'])
  </code></pre>