Today is rung 9 of 14 on this topic
Adding and subtracting with regrouping
Can do: Adds and subtracts two- and three-digit numbers with carrying and borrowing, and can explain why it works.
Why it matters
This is the first algorithm children are asked to trust. If they learn it as a magic rule, it collapses the moment a zero shows up in the middle.
What it looks like when it's wobbly
Subtracts the smaller digit from the larger regardless of position (63 − 28 = 45), or forgets to carry.
⏱️ Two-minute check
"Work out 63 − 28 and 45 + 37." Then: "Show me why you crossed that out."
Solid looks like: Both correct, plus some account of exchanging a ten for ten ones.
The full activity for this topic · 15 minutes
Trade the Bundle
- 1Build the first number with bundles: 63 is six tens and three ones.
- 2Try to take away eight ones. There aren't enough — this is the moment to pause on.
- 3Physically break open a ten. Now there are thirteen ones. Do the subtraction with objects.
- 4Only now write it down, and match each written step to what their hands just did.
- 5Do one where nothing needs trading, so they don't start regrouping automatically every time.
If it's too hard
Two-digit only, and do every problem with the objects first.
If it's too easy
Three digits with a zero in the middle — 305 − 128 — which is where most children break.
Say this
"You can't take 8 from 3. So go next door and borrow a ten — break it open into ten ones."