have an xml file like
<Month>
   <Date day=b1b>
       <Session task=btask1b>
           <Notes hours-worked=b1.5b/>
       </Session>
       <Session task=btask2b>
           Notes hours-worked=b5b/>
       <Session task=btask1b>
           <Notes hours-worked=b3b/>
       </Session>
   </Date>
   <Date day=b2b>
       ....
   </Date>
</Month>
For the month, I want to sum all the hours-worked for each task:

I have tried several expressions, but failed.
This gives me the total hours worked on all
tasks rather than each individual task:
<xsl:for-each-group select="Date/Session" group-by="@task">
     <xsl:for-each select=".">
       <fo:block xsl:use-attribute-sets="subdiv2">
         <xsl:value-of select="@task"/>
         <xsl:text> (</xsl:text>
         <xsl:value-of
select="format-number(sum(../../Date/Session/Notes/@hours-worked),

         <xsl:text>) </xsl:text>
       </fo:block>
   </xsl:for-each>
</xsl:for-each-group>
How do I restrict the total to just the hours-worked on each specific task?

You have to work with those sessions that are in
the current group.  Rather, you are looking at
all sessions and getting the total sum.

  <xsl:value-of
select="format-number(sum(current-group()/Notes/@hours-worked),'.00')"/>

You've already correctly created the groups, you
just aren't using them having created them.

BTW, the <xsl:for-each select="."> is meaningless
and does nothing except render your context list
to be only a single item which is not
useful.  Although you don't need the set of
groups in your solution, not having that
<xsl:for-each> would give you more information
about the current group within the set of groups.

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