Today is rung 9 of 11 on this topic
Decimal operations and powers of ten
Can do: Adds, subtracts, multiplies and divides decimals, and knows what multiplying by 10 or 0.1 does.
Why it matters
Decimals are where place value is stress-tested. Errors here appear in money, measurement and science for years.
What it looks like when it's wobbly
Lines decimals up by the last digit instead of the point, and thinks multiplying by 10 "adds a zero" — which fails immediately for 2.5.
⏱️ Two-minute check
"3.4 + 12.75? 2.5 × 10? 4.2 ÷ 100?"
Solid looks like: 16.15, 25, 0.042 — and they can say why 2.5 × 10 isn't 2.50.
The full activity for this topic · 20 minutes
Slide the Digits
- 1Write a number on a place value chart and physically slide the digits one column left for ×10.
- 2Say it correctly: the digits move, the point doesn't. This kills the add-a-zero rule permanently.
- 3For addition, always write the numbers with the points lined up and fill gaps with zeros.
- 4Estimate before multiplying: 4.2 × 3 must be around 12.
- 5Use money and measurement so the answers can be sanity checked.
If it's too hard
Addition and subtraction only, with the points lined up on squared paper.
If it's too easy
Divide a decimal by a decimal, and multiply two decimals with an estimate first.
Say this
"The point doesn't move — the digits do. Slide them one column and read it again."