Average

      <xsl:stylesheet
                  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                  version="2.0">
            <xsl:output  method="xml"
                        indent="yes"
                        omit-xml-declaration="yes"/>

            <xsl:template  match="/aaa">
                  <bbb>
                        <xsl:value-of  select="avg(1 to 99)"/>
                  </bbb>
                  <ccc>
                        <xsl:value-of  select="avg(a)"/>
                  </ccc>
                  <ddd>
                        <xsl:value-of  select="avg(b)"/>
                  </ddd>
            </xsl:template>

      </xsl:stylesheet>

      <aaa>
            <a>5</a>
            <a>3</a>
            <a>7</a>
            <a>2</a>
      </aaa>

      <bbb>50</bbb>
      <ccc>4.25</ccc>
      <ddd/>