My friend Rodrigo lives in apartment number 28 that is on the 6th floor of the building. Going up the stairs I see that there are no apartments on the ground floor and that each of the floors has the same number of doors.
How many apartments are on each floor?
Each floor has 5 apartments.
If we do the entire division of my friend's apartment number by the floor number 28/6 we get 4 with a remainder of 4. So I see there have to be 4 or more apartments per floor. If there were 4, apartment 28 would be on the 7th floor. If there were 6 apartments per floor, apartment 28 would be on the 5th floor so there have to be 5 apartments on each floor.