In and At
Rule: in is used to describe the physical location of something; at is used when we refer to an address, place, building, etc. It is also used in cases where the location is not an issue; rather, what we do there is important (school, park, mall, hospital, etc.).
Incorrect Usage: (a) Henry owns a mansion at New York.
(b) The British Prime Minister stays in 10 Downing Street.
Correct Usage: (a) Henry owns a mansion in New York.
(b) The British Prime Minister stays at 10 Downing Street.