Today I need to do my laundry and then go to the gym so I will be going down 5 floors and then I will have to come back up 9 floors to get to the gym. So I will have gone -5+9=4. My final location would be 4 floors above my apartment.
The difference between the highest and lowest floor can be calculated by doing 20-0=20 because there are 20 floors above ground and 0 floors below ground so the difference between the highest floor and the lowest floor is 20.
Since I am able to walk down 3 pairs of stairs per minute, after 5 minutes I would have walked down 15 pairs of stairs in 5 minutes. So I have walked -3 X 5= -15 floors from floor 20 of the apartment. My final location would be 7 floors below my apartment or -7.