let $islandsInLake := //island[@lake != ""]
for $cont in //continent
return <continent name="{data($cont/name)}">
<total_lake_area>{
    let $countrysum := (
        for $country in //country
        where data($country/encompassed/@continent) = data($cont/@id)
        let $countryPercentage := data(($country/encompassed[@continent = $cont/@id]/@percentage) * 0.01)
        let $lakesWithIslandArea := (
            for $lake in //lake[@id = $islandsInLake[@country = $country/@car_code]/@lake]
            return (data($lake/area)) * $countryPercentage
        )
        return $lakesWithIslandArea
    )
    return sum($countrysum)
}</total_lake_area>
</continent>