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.