title: How to sum the values in a column conditioned by the values in another column?
body: <p>I have two columns in a data frame, called <code>x</code> and <code>total</code>.
  How can I calculate the sum of all cells in the <code>total</code> column where the value in <code>x</code> column is more than a specific number (let's say <code>Y</code>)?</p>