Adding and subtracting decimals works just like whole numbers, with one rule that matters most: line up the decimal points. That puts tenths under tenths and hundredths under hundredths.
Try it with your own problem ↓
The steps
- Write the numbers in a column with the decimal points directly above each other.
- Fill the empty places on the right with zeros so every number has the same number of decimal places. A whole number has its decimal point at the end: 10 = 10.00.
- Add or subtract as you would with whole numbers, starting at the right, carrying or borrowing as needed.
- Bring the decimal point straight down into the answer.
A worked example
Work out 25.4 + 3.75.
25.40
+ 3.75
-------
29.15
- Line up the points and write 25.4 as 25.40 so both have two decimal places.
- Hundredths: 0 + 5 = 5.
- Tenths: 4 + 7 = 11. Write 1 and carry 1 to the ones.
- Ones: 5 + 3 = 8, plus the carried 1 is 9.
- Tens: 2 + 0 = 2.
- Put the decimal point straight down: 29.15.
25.4 + 3.75 = 29.15. Estimate: 25 + 4 = 29. ✓
Common mistakes
- Lining up the right-hand digits instead of the points. 25.4 + 3.75 is not 25.4 + 37.5 or 2.54 + 3.75.
- Forgetting the placeholder zeros. Without 25.40, it is easy to subtract the wrong digits.
- Misplacing the decimal point in the answer. It goes straight down from the others.
- Treating a whole number as having no point. 10 is 10.00 for subtracting 2.37.
Questions people ask
Why do the decimal points need to line up?
Because only digits of the same place value can be added. Tenths go with tenths.
How do I subtract a decimal from a whole number?
Write the whole number with a decimal point and zeros: 10 − 2.37 is 10.00 − 2.37 = 7.63. Borrow across the zeros.
Does adding zeros at the end change the number?
No. 25.4 and 25.40 are equal. The zeros are just placeholders.
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