When you add decimals, line up the decimal points, not the last digits. Write the numbers one above the other so the points sit in a straight line, then add each column from the right just like whole numbers. Bring the decimal point straight down into the answer. It helps to pad with zeros at the end so both numbers have the same length: 2.5 becomes 2.50, then 2.50 + 0.25 = 2.75. The extra zero always goes at the end, never straight after the point.
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Equal parts, equal wholes
2/4 = 4/8
Both bars are the same length. Find a division of the second bar that lines up exactly with the coloured edge.
Words to know
- decimal point
- the dot separating whole numbers from parts
- column
- digits with the same place value, stacked to add
- place value
- the value a digit has from its position
Worked examples
2.5 + 0.25
- 1.Line up the points: 2.50 above 0.25.
- 2.Add each column: 2.75.
= 2.75
1.2 + 0.35
- 1.Line up the points: 1.20 above 0.35.
- 2.Add each column: 1.55.
= 1.55
3.4 + 1.25
- 1.Line up the points: 3.40 above 1.25.
- 2.Add each column: 4.65.
= 4.65
For parents
Method: Stack the numbers with the decimal points aligned, then add columns.
What to say: Check the points make one straight vertical line before adding.
Common mistakes:
- Right-aligning the last digits and dropping the whole-number part (→ 0.75).
- Padding a zero straight after the point (2.5 → 2.05) instead of at the end (2.50).