Math · Stage 3
Model a transfer with before-and-after bars using the unchanged total
When one person gives to another, the two bars change — but tip both piles together and the combined total never moves.
✦ Math📘 Stage 3 · S3⏱ ~12 min🧒 Ages 7–8★ Free to start
I can model a transfer between two people with before-and-after bars, finding each new amount and using the unchanged combined total to reason and check.
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Kavi's Marble Swap 🔵
Kavi has 50 marbles 🔵 and Pip has 30 — that's 80 altogether. Kavi slides 20 marbles across to Pip. Now Kavi has 30 and Pip has 50 — but tip both piles into one bowl and there are still 80! "Giving doesn't make marbles vanish," hoots Kavi. "The two piles change, but the whole stays the same — and that fixed total helps me check my work."
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What your child will learn
1Two piles, one combined totalDraw the two people as two parts of one whole — the combined total. Here a small bond shows it: 5 (Kavi) + 3 (Pip) = 8. Drag to split the whole and watch the two parts always rebuild it.
2Each person is a bar — before the swapDraw before first. Kavi's bar is 50 marbles, Pip's bar is 30. Line them up and read the two lengths.
3The transfer: one drops, one rises, total staysKavi gives 20 to Pip. The giver's bar goes DOWN 20; the getter's bar goes UP 20. Tap each card and check the combined total.
4Work the after amounts, then checkFind the giver's new amount by subtracting. Kavi gives 20, so tap Step → to work 50 − 20.
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A peek inside
Kavi has 50 🔵 and Pip has 30. Kavi gives 20 to Pip. Find each new amount and check the total.
Total before = 50 + 30 = 80. Giver: 50 − 20 = 30. Getter: 30 + 20 = 50. Check: 30 + 50 = 80 ✓
Try this at home: Kavi has 50, Pip has 30. Kavi gives 20 to Pip. New Kavi = __ · New Pip = __ · Total = __
Ready to play and learn?
Start the free interactive lesson, or print the handout for screen-free play.
Part of Math · Stage 3 · S3 on KAVI Academy — a secular, whole-child learning platform for ages 5–12.