The ground floor is the starting point from which every other floor is counted. Floors above it need some presses of ‘+’, floors below need some presses of ‘–’, and the ground floor itself needs none.
Why this matters: the ground floor is the reference floor. That is exactly the job of 0 among the numbers — it separates the positive numbers from the negative numbers and belongs to neither group.
Q2.
Number all the floors in the Building of Fun.
Answer
Start at the Welcome Hall as Floor 0. Count upwards with + and downwards with –.
Every floor of Bela's Building of Fun with its number. The brown line is the ground; above it the numbers are positive, below it they are negative.
Floor number
What is there
Button press from the ground floor
+ 6
Space
+ + + + + +
+ 5
Sports Centre
+ + + + +
+ 4
Ice Cream
+ + + +
+ 3
Book Store
+ + +
+ 2
Art Centre
+ +
+ 1
Food Court
+
0
Welcome Hall (ground floor)
no press at all
– 1
Toy Store
–
– 2
Video Games
– –
– 3
Cinema
– – –
– 4
Ghost House
– – – –
– 5
Dinosaur
– – – – –
The neat part: the floor number and the button press are the same number. + 3 is the Book Store and the movement of three floors up; – 3 is the Cinema and the movement of three floors down.