adding variables

I want to have an offset stored so that I can move
around all off my components based on this offset. I
have created a variable:

<xsl:variable name="y_offset">
  <xsl:value-of select="20"/>
</xsl:variable>

and want to add to the offset. Something to the effect
of:

<path d="M 35, {$y_offset} + 25 ....

The output of this command is  20 + 25 though and not
45 like I would like it to be. I can't use the
<xsl:valueof> because it is an attribute. 

Any recommendations on how to do this or a better way
to do this would be greatly appreciated.

Thanks,

Bryan

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