In recent Mercer’s 2015 annual quality-of-living survey, New Delhi ranks at number 154 out of 230 cities due to bad air quality and pollution. The World Health Organization ranked New Delhi as the world’s worst polluted city in 2014 among about 1,600 cities the organization tracked around the world.
How many cities were studied in Mercer's 2015 quality of living survey? (If the question is unanswerable, say "unanswerable")
230