title: Calculated cells in Telerik RadGrid's GridGroupFooterItem
body: <p>I'm populating a Telerik RadGrid with data from a DataTable (<code>System.Data.DataTable</code>). As I'm supporting someone else's application, I can't use any other data source or display control.</p>
  
  <p>In my grid I have three columns: let's say they are Widgets Produced (column A), Faulty Widgets (column B) and Faultiness Proportion (column C). The database query provides these for me and does the calculation C = B / A.</p>
  
  <p>I have a totals row (a Telerik <code>GridFooterItem</code>) at the bottom of the grid. In columns A and B Telerik calculates the totals for those columns for me. We can't calculate the correct value for the 'total' of column C from column C alone: we have to populate it with (the sum of B) / (the sum of A).</p>
  
  <p>I've managed to do this by handling the <code>DataBound</code> event of the RadGrid and manually populating the cells in the footer. (I had to catch the <code>GridFooterItem</code> in the <code>ItemCreated</code> event, then put values in it in the <code>DataBound</code> event, after it had automatically calculated the totals for A and B for me.) This feels pretty hacky - maybe there's a better way...?</p>
  
  <p>Anyway, the important bit is this:</p>
  
  <p>My grid is split into groups so I also need to populate column C in the <code>GridGroupFooterItem</code>s. I can't get my hacky technique to work in this case: I'm finding the footer cell I want with <code>myGridFooterItem["WidgetsProduced"]</code>, but I can't get the group footer cells with <code>myGridGroupFooterItem["WidgetsProduced"]</code> - it just isn't a dictionary.</p>
  
  <p>I've tried using <code>myGridGroupFooterItem.Cells[]</code>, but this <code>TableCellCollection</code> contains a couple more cells than I'd expect, so accessing them by integer index feels a tad ropey (especially as this is a user-defined report, so the columns may be in any order).</p>
  
  <p>So: how do I populate these cells with my calculation?</p>