Add and subtract decimals by place value
Keep wholes, tenths and hundredths in their own places. Extra zeros can help you see those places without changing a value.
One ribbon is 0.6 metre long and another is 0.75 metre long. To join their lengths, count them in the same unit. Six tenths is sixty hundredths, so write 0.6 as 0.60. Both lengths now use hundredths.
Add lengths without mixing their units
Find 0.6 + 0.75 metres.
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Give the lengths the same place values
The zero adds no length. It shows that the first ribbon has sixty hundredths.
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Add the hundredths
One hundred hundredths make one metre. Thirty-five hundredths remain, so the total is 1.35 metres.
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Use the same alignment to subtract
Think of 420 hundredths minus 35 hundredths. The result is 385 hundredths, or 3.85.
Line up the places, not the last digits
The decimal point marks where the wholes end. Put the points in one column when you write a sum. This keeps tenths with tenths and hundredths with hundredths. The table names each place by its value in metres.
In your head, you can use the same idea by naming one unit. For 3.4 − 0.27, count hundredths: 340 − 27 = 313. So the difference is 3.13.
| Metres | 1 | 0.1 | 0.01 |
|---|---|---|---|
| 0.60 | 0 | 6 | 0 |
| 0.75 | 0 | 7 | 5 |
| 1.35 total | 1 | 3 | 5 |
Check the size of the answer
The two ribbons together must be longer than either one. About 0.6 + 0.8 is 1.4, so 1.35 is a sensible total. A result such as 0.135 would be far too small.
Try it yourself
Work out an answer and explain your reasoning. Then check the explanation.
Regroup across a whole
Find the total length. Choose a shared unit if it helps.
Show the answerHide the answer
Add eighty-five hundredths and seventy hundredths. You get 155 hundredths, or 1.55.
Subtract different decimal lengths
Find how much length remains. Explain why the decimal places stay aligned.
Show the answerHide the answer
Work out 3.40 − 0.27. To check, add 0.27 back to 3.13.
A near-whole shortcut
For 2.99 + 1.6, add 3 instead, then take back 0.01. You get 4.59. Use this when it saves effort; matching place values still works when no shortcut is clear.
Try lengths with new decimal places
Choose addition or subtraction. The game varies the number of places. Use place value, then check whether the size of your answer fits the sum.
2-minute practice · keep going after mistakes · opens in a new tab