Rename the decimal as tenths or hundredths so it becomes a whole number. 0.4 is 4 tenths, so 0.4 × 3 is 12 tenths, which is 1.2. 2.35 is 235 hundredths, so 2.35 × 4 is 940 hundredths, which is 9.4. Estimate first to check where the point goes: 2.35 × 4 is a bit more than 2 × 4 = 8.
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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
- tenths
- parts of one split into 10, the first place after the point
- hundredths
- parts of one split into 100, the second place after the point
- estimate
- a quick rough answer used to check
Worked examples
0.4 × 3 = ?
- 1.0.4 is 4 tenths.
- 2.4 tenths × 3 = 12 tenths = 1.2.
= 1.2
2.35 × 4 = ?
- 1.2.35 is 235 hundredths. 235 × 4 = 940.
- 2.940 hundredths = 9.4.
= 9.4
1.25 × 12 = ?
- 1.1.25 is 125 hundredths. 125 × 12 = 1500.
- 2.1500 hundredths = 15.
= 15
For parents
Method: Multiply in tenths or hundredths
What to say: What is the decimal in tenths or hundredths? About how big should the answer be?
Common mistakes:
- Multiplies the whole-number part and the decimal part separately and writes them side by side without regrouping (3.6 × 7 → 21.42).
- Multiplies only the whole-number part and copies the decimal part unchanged (2.35 × 4 → 8.35).
- Multiplies the digits as whole numbers and leaves out the decimal point (0.4 × 3 → 12).
- Places the decimal point one place too far left, as if the answer needed an extra decimal place (0.4 × 3 → 0.12).