PREFIX dc: <http://purl.org/dc/elements/1.1/>
PREFIX ns: <http://example.org/ns#>
SELECT ?title ?price WHERE {
  ?x ns:price ?p .
  ?x ns:discount ?discount .
  BIND( ( ?p * ( "1"^^<http://www.w3.org/2001/XMLSchema#integer> - ?discount ) ) AS ?price )
  FILTER ( ( ?price < "20"^^<http://www.w3.org/2001/XMLSchema#integer> ) )
  ?x dc:title ?title .
}