Primary 4 maths · Decimals · About 12 minutes
Adding and subtracting decimals
Add and subtract decimals with up to two decimal places
Start the interactive lesson →Open practice activity
What you will learn
- Add and subtract decimals with up to two decimal places
- Match digits by place value before calculating
- Check an answer using the inverse operation
Models used: place-value charts, hundred grids, number sentences, stories.
Before you start
Match the place values
Twenty-five hundredths means 25 little squares.
The whole grid has 100 equal squares. One full row is 10 hundredths, or one tenth.
Twenty-five hundredths means 25 little squares.
0.25
One small square is one hundredth of the whole. Ten small squares make one tenth, so the same shaded amount can be described in more than one way.
25/100 = 0.25 = 25%
The whole grid has 100 equal squares. One full row is 10 hundredths, or one tenth.
Add 30 hundredths first. There are now 55 shaded squares.
0.25 + 0.30 = 0.55
One small square is one hundredth of the whole. Ten small squares make one tenth, so the same shaded amount can be described in more than one way.
55/100 = 0.55 = 55%
The whole grid has 100 equal squares. One full row is 10 hundredths, or one tenth.
Add the remaining 8 hundredths. The whole has 63 shaded squares.
0.25 + 0.38 = 0.63
One small square is one hundredth of the whole. Ten small squares make one tenth, so the same shaded amount can be described in more than one way.
63/100 = 0.63 = 63%
The whole grid has 100 equal squares. One full row is 10 hundredths, or one tenth.
Taking away those same 38 squares brings us back to 25.
0.63 − 0.38 = 0.25
One small square is one hundredth of the whole. Ten small squares make one tenth, so the same shaded amount can be described in more than one way.
25/100 = 0.25 = 25%
Why must the columns match?
Adding means combining amounts of the same size. Four tenths and five hundredths are different sizes, so they cannot be combined until they are written in the same units: 0.40 and 0.05 are 40 and 5 hundredths.
40/100 = 0.4 = 40%
Worked example
6.15 − 2.75
Write both to the same number of decimal places.
6.15 − 2.75
Think in hundredths.
615 hundredths − 275 hundredths
615 − 275 = ?
340
340 hundredths is 3 ones and 4 tenths.
Ones3Tenths4Hundredths0Thousandths03.4 = 3 ones + 4 tenths
So the answer is 3.4.
6.15 − 2.75 = 3.4
Check by adding back.
3.4 + 2.75 = 6.15 ✓
Common mistakes to discuss
- Lining the numbers up on the right. 3 + 0.45 is not 3.45 written as 348. Ones go under ones and hundredths under hundredths, whatever the length.
- Forgetting to regroup ten hundredths as one tenth. 0.07 + 0.05 is 12 hundredths, which is 1 tenth and 2 hundredths: 0.12.
Check your understanding
- work out 3.40 + 2.75 and 6.15 − 2.75
- explain why digits must sit under the same column
- check a decimal answer by adding back
Explain your reasoning to an adult, then try a fresh question in the interactive lesson. A result from one attempt is not proof of lasting mastery.
Try this lesson →Related Primary 4 lessons
- Tenths, hundredths and thousandthsRead the value of each digit in a decimal · About 12 minutes
- Use decimal place value to three decimal placesUse decimal place value to three decimal places · About 10 minutes
- Compare and order decimalsCompare and order decimals · About 10 minutes
- Convert fractions to decimals when the denominator is a factor of 10 or 100Convert fractions to decimals when the denominator is a factor of 10 or 100 · About 10 minutes
- Round decimals to a whole number, one or two decimal placesRound decimals to a whole number, one or two decimal places · About 10 minutes
- Add and subtract decimals up to two decimal placesAdd and subtract decimals up to two decimal places · About 10 minutes