Decimals: where the point goes
The digits in the answer are right; the decimal point is one place out. It happens most when multiplying or dividing a decimal by a whole number, and when adding a decimal with one decimal place to one with two.
What it looks like
Sofia buys 3 bottles of juice at $1.45 each. How much does she pay?
- Work as whole numbers: 145 × 3 = 435
- 1.45 has two decimal places, so the answer has two: 4.35
- She pays $4.35
Why $43.50 is wrong: The point was put back one place too far to the right, giving ten times the real cost.
Why children do it
- Multiplying is done as whole numbers (145 × 3) and the point is put back by feel rather than by counting places.
- When adding, numbers get lined up by their last digit instead of by the decimal point.
- Dividing by 10, 100 or 1000 moves the point a different number of places each time, and the count slips.
How to fix it at home
- After every decimal multiplication, ask: "Is that a sensible amount?" Three drinks cannot cost $43.
- Count the decimal places in the question out loud, then count them in the answer.
- For adding and subtracting, write the numbers with the decimal points in one vertical line and fill empty places with 0.
Questions parents ask
Is this the same as a unit conversion mistake?
They look alike, because 1.5 kg to grams also moves the point. The habit here is the point moving during arithmetic itself, with no units involved.
Will estimating help?
Yes. "About 3 × $1.50 = $4.50" takes two seconds and rules out $43.50 at once.
How does Knowbotic catch it?
The answer with the point one place out is a known trap on decimal questions, so the app names the habit and practises it until it is fixed.
Where it shows up
Other careless mistakes
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