Adding and subtracting decimal numbers

The golden rule: aligning the decimal points

When adding or subtracting decimal numbers, the most important rule is to align the decimal points one below the other, just as you would align the units when working with whole numbers.

Example: 12.5 + 3.47

  12,50
+  3,47
--------
  15,97

We have padded 12.5 with a zero (12.50) so that both numbers have the same number of digits after the decimal point, which makes the calculation easier.

Subtraction with ‘borrowing’

Example: 20 – 4.35

  20,00
-  4,35
--------
  15,65

Here too, we write 20 as 20.00 to ensure the decimal points are properly aligned before subtracting.

Practical example

At the supermarket, you buy a loaf of bread for 1.20 euros and some cheese for 4.85 euros. You pay with a 10-euro note.

• Total cost of purchases: 1.20 + 4.85 = 6.05 euros
• Change given: 10.00 - 6.05 = 3.95 euros

A common pitfall to avoid

The most common mistake is to align the numbers on the right (as with whole numbers) instead of aligning the decimal points. If you write 12.5 + 3.47 without aligning the decimal points, you risk adding a tenth to a hundredth and getting the wrong answer. Always check that the decimal points are lined up correctly before calculating.