Question: What is the average Washington DC teacher salary?
SPARQL:
 select avg(xsd:decimal(?amount)) as ?result from <http://linkedspending.aksw.org/dc-city-salaries> { ?obs qb:dataSet ls:dc-city-salaries. ?obs lso:dc-city-salaries-amount ?amount. ?obs lso:dc-city-salaries-position "Teacher". }