Math · Stage 3
Test divisibility by 3 and 9 from the digit sum
Add the digits — if the sum divides by 3, so does the number; if the sum divides by 9, so does the number.
✦ Math📘 Stage 3 · S3⏱ ~10 min🧒 Ages 7–8★ Free to start
I can decide whether a number divides exactly by 3 or 9 by adding its digits and testing the digit sum.
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Kavi's Digit Trick 🔢
Kavi looks at the big number 72 and doesn't divide at all! "Watch this China-fast trick," hoots Kavi. "Just add the digits: 7 + 2 = 9. Nine is a times-table-of-9 number, so 72 divides by 9 — no long division needed!"
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What your child will learn
1Count by 3First, know the 3s. Counting in groups of 3 lights up every number that divides by 3: 3, 6, 9, 12, 15 …
2Add the digits, then test
3Where the digits come fromThe digit trick just reads the digits. The number 72 is 7 tens and 2 ones — so its digits are 7 and 2. Add them: 7 + 2 = 9.
4The 9s hide a patternNo pictures now — just look. Counting in 9s: 9, 18, 27, 36, 45, 54, 63, 72, 81, 90.
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A peek inside
Does 63 divide by 9? Use the digit trick.
6 + 3 = 9 → 9 is a 9s number
Try this at home: Add the digits of 51: 5 + 1 = __ · Does 51 divide by 3? __
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.