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 →Practise: Decimal builders
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.
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