How do I sort using formula?

Hi,

I created a table with the following fields:
1. Vendor's code
2. Vendor's name
3. Quantity of orders: count(orders)
4. Total of orders: sum(orders/total)

When the user clicks on the header of the table he can sort that column. In 
the xls I wrote:

<td title="Click here to sort this column" align="center"><div class="header" 
onClick="sort('code', 'number')">Code</div></td>

And to sort:

   <xsl:apply-templates>
        <xsl:sort select="*[name() = $FieldSort]" data-type="{$FieldType}" />
   </xsl:apply-templates>


It works to vendor's code and name, because "code" and "name" are tags in 
the xml. But how can I sort the quantity and total of orders?

I am using MSXML.

Thanks in advance,
Vanessa

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