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You may be familiar with another meaning of subtraction which is related to
comparison or making quantities equal.
For example, I have Rs. 10 with me and my sister has Rs. 6. Now, I can ask the
question: ʻHow much more money should my sister get in order to have the same
amount as me?ʼ We can write this in two ways: 6 + ? = 10 Or 10 – 6 = ? Here, we see
the connection between ‘finding the missing number to be added’ and subtraction.
For subtraction of positive and negative numbers, we will use this meaning of
subtraction as ‘making equal’ or ‘finding the missing number to be added’
We can write this as an expression using subtraction:
Target Floor – Starting Floor = Movement needed.
In the above example, the starting floor is + 2 (Art Centre) and the target floor is + 5.
The button press to get to + 5 from + 2 is + 3. Therefore, (+ 5) – (+ 2) = + 3
Recall the connection between addition and subtraction. For 3+ ?=5, we can find the
missing number using subtraction: 5–3=2. That is, subtraction is the same as finding
the missing number to be added.
We know that Starting Floor + Movement needed = Target Floor If the movement
needed is to be found, then, Starting Floor + ? = Target Floor So, Target Floor –
Starting Floor = ? = Movement needed .
More examples:
a. If the Target Floor is – 1 and Starting Floor is – 2, what button should you press?
You need to go one floor up, so, you should press + 1. Expression: (– 1) – (– 2) =
(+1)
b. If the Target Floor is – 1 and Starting floor is +3, what button should you press?
You need to go four floors down, so, you should press – 4. Expression: (– 1) – (+ 3)
= (– 4)