This is a terribly worded question. Here is how it should have been phrased -

Write a query to find the difference between the total number of cities and the unique number of cities in the table STATION.

I don't understand why he says "Let's say" and gives all NUM etc.

Here problem statement itself is a problem.

SELECT COUNT(CITY) - COUNT(DISTINCT CITY) FROM STATION;

worked for me.

select (count(CITY)-COUNT(DISTINCT CITY)) from STATION;

Here is Oracle solution from my HackerrankPractice repository:

SELECT COUNT(CITY) - COUNT(DISTINCT CITY)
FROM STATION;

Feel free to ask if you have any questions :)