title: Calculating Running Balance column
body: <p>I need help with R and the data below. For each date there is a value  (column A) and i need to calculate the Balance column which is total less each lines value in column A. 406 is the total of column A.</p>
  
  <p>I currently have this data in a data.table.</p>
  
  <pre class="lang-none prettyprint-override"><code>Dates        A        Balance
                        406.00
  30/12/2016   100.00   306.00 
  31/03/2017   101.00   205.00 
  30/06/2017   102.00   103.00 
  29/09/2017   103.00   -   
  </code></pre>
  
  <p>Thanks</p>