3 Addition and subtraction on the number line
Watch the video below to see how you can use a number line to help you with addition and subtraction involving negative numbers.
Download this video clip.Video player: swm_1_s7_addition_subtraction_number_line.mp4
Transcript
NARRATOR:
The number line can be used to show how calculations can be carried out with negative numbers. First, you'll have a reminder of how this works with positive numbers, by looking at the sum 2 plus 4. To add 4 onto 2, you need to move four places to the right of 2, giving 6.
The same method of moving to the right works even if the starting number is negative. For example, suppose an account is overdrawn by 5 pounds. The account balance will be recorded as minus 5. If the account holder then paid 8 pounds into the account, 5 pounds of this would pay off the debt and 3 pounds will be recorded as the new balance. That's like saying minus 5 plus 8 equals 3. This is verified on the number line.
Now let's consider subtraction-- again, first with positive numbers-- for example, 6 minus 4. The starting number is 6, but this time, you move 4 units to the left to get the answer, which is 2. To subtract, you move to the left on the number line.
Similarly, if an account is overdrawn by 2 pounds-- so that's a balance of minus 2-- and then the account holder withdrew another 3 pounds, the account will be overdrawn by 5 pounds after that. This is represented on the number line, by starting at minus 2 and then moving 3 units to the left to get the answer minus 5. This is written as, minus 2 take away 3 equals minus 5.
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See how you get on in this next activity.
Activity 3 Working with negative numbers
Timing: Allow approximately 10 minutes
Try the following examples. Start by drawing your own number line from –10 to + 10.
- a.3 − 7
- b.(–2) + 5
- c.(–3) − 6
Well done for completing your first calculations using negative numbers. These examples all involved subtracting or adding positive numbers to a negative number. Things become slightly more challenging when looking at adding and subtracting negative numbers, but again you’re going to call in your trusty number line to help.