请问怎么用XQuery去计算每个元素分别有多少个 子元素，然后加起来 求他们的平均数
<MovieList>
<Movie ID= "M1">
<Version id= "M101">
<Name>...e la vita continua</Name>
<Year>1984</Year>
</Version>
<Version id= "M102">
<Name>...und das Leben geht weiter</Name>
<Year>1986</Year>
<Country>West Germany</Country>
</Version>
<Version id= "M103">
<Name>Et la vie continue</Name>
<Year>1984</Year>
<Coutry>France<Country>
</Version>
</Movie>
<Movie ID= "M2">
<Version id= "M201">
<Name>Til Death Do Us Part</Name>
<Year>2006</Year>
</Version>
<Version id= "M202">
<Name>John Waters Presents Love You to Death</Name>
<Year>2006</Year>
<Country>Australia</Country>
<Title>DVD title</Title>
</Version>
<Version id= "M203">
<Name>Love You to Death</Name>
<Year>2006</Year>
<Title>Canada: English title</Title>
</Version>
<Version id= "M204">
<Name>Love You to Death<Name>
<Year>2006</Year>
</Version>
</Movie>