XSL Total variable

[mjs87]

Hi, I am fairly new to XML/XSL. I am trying to sum a variable that is within a for-each loop. The $subtotal variable is working out the total price for each product ordered on a invoice. Once all the products have been output I need to calculate the overall total price. I have tried to create a variable and increment it on each loop but you cannot change a variable once it has been set from what I have read. below just shows where I have just tried to sum the $subtotal variable...any ideas would be appreciated...thanks in advance

 

<xsl:for-each select="DeliveryDetails/ProductOrder">
<tr align="center">
  <td width="150"><xsl:value-of select="Description" /></td>
   <td width="150"><xsl:value-of select="ReferenceNo" /></td>
  <td width="100"><xsl:value-of select="Quantity" /></td>
  <td width="100">£<xsl:value-of select="Price" /></td>
  <td width="100">£  <xsl:variable name="subtotal" select="Quantity*Price" /><xsl:value-of select="$subtotal" /></td>
</tr>
  </xsl:for-each>

<xsl:value-of select="sum($subtotal)"/>