Subtraction to find which button to press
In earlier classes, we understood the meaning of subtraction as 'take away'. For example, there are 10 books on the shelf. I take away 4 books. How many are left on the shelf?
We can express the answer using subtraction: 10 – 4 = 6 or 'Ten take away four is six.'
You may also be familiar with another meaning of subtraction which is related to comparison or making quantities equal.
For example, consider this situation: I have ₹10 with me and my sister has ₹6. Now, I can ask the question: 'How much more money should my sister get in order to have the same amount as me?'
We can write this in two ways:
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'.
Evaluate the following from above perspective
In general, when there are two unequal quantities, subtraction can indicate the change needed to make the quantities equal. Subtraction shows how much the starting quantity should change in order to become the target quantity. In the context of different floor levels, what is the change required to reach the Target Floor from the Starting Floor? Notice that the change needed may be positive (for an increase) or negative (for a decrease).
Your starting floor is the Art Centre and your target floor is the Sports Centre. What should be your button press?
You need to go three floors up, so you should press + 3. 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,
Explanation
Recall the connection between addition and subtraction. For
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
a. If the Target Floor is –1 and Starting Floor is –2, what button should you press?
Answer: You need to go one floor
Expression:
b. If the Target Floor is – 1 and Starting floor is +3, what button should you press?
Answer: You need to go four floors
Expression:
c. If the Target Floor is +2 and Starting Floor is –2, what button should you press?
Answer: You need to go four floors
Expression:
Complete these expressions. You may think of them as finding the movement needed to reach the Target Floor from the Starting Floor.
a.
b.
c.
d.
e.
f.
g.
h.
i.
j.