Show the number of cities in counties that have a population more than 20000.
SELECT count(*) FROM city WHERE county_ID IN (SELECT county_ID FROM county_public_safety WHERE population  >  20000)
cities in counties with a population more than 20000 refers to a subquery condition population > 20000 in the county_public_safety table; number of such cities refers to COUNT(*) from city table where city's county_ID matches those in the subquery.