Adding and subtracting negative numbers
Maths · Key Stage 2 · The School
A number line with two directions
You have already met negative numbers on a number line. Now use that line to calculate with them. Adding moves you to the right. Subtracting moves you to the left. That rule holds true whether you start on the positive side or the negative side. Drawing the line, and marking your starting point, makes every calculation like this far harder to get wrong.
Adding a negative number
Work out minus 2 add 5. Start at minus 2 on the number line. Adding moves you right, so move 5 steps right. You land on 3. Adding always moves right, no matter where you began, and no matter whether the number you started at was negative. The direction never changes. Only your starting point on the line changes from one question to the next.
Subtracting a negative number
Work out 3 take away minus 2. This looks strange at first. Two negatives together actually turn into an addition, so it becomes 3 add 2, which is 5. Subtracting a negative number always turns into adding its positive twin. That single rule solves every case like it. Two minus signs sitting side by side always cancel each other out this way.
Solving a temperature problem
The temperature is minus 4 degrees. It rises by 9 degrees. Start at minus 4 on the number line. A rise means adding, so move 9 steps right. Minus 4 add 9 lands on 5. The new temperature is 5 degrees. Drawing the jump on a number line makes the direction impossible to get wrong.