Math · Stage 5
Multiply two 2-digit numbers with equal tens and units summing to ten
When two 2-digit numbers share a tens digit and their ones add up to 10, one glance gives the product — a lightning-fast Chinese mental-math shortcut, no column working needed.
✦ Math📘 Stage 5 · S5⏱ ~12 min🧒 Ages 9–10★ Free to start
I can multiply two 2-digit numbers with the same tens digit and units summing to ten.
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Kavi’s Lightning Multiply ⚡
The class is racing to work out 23 × 27. Pencils scratch… but Kavi just blinks and hoots “621!” “I spotted a secret pattern,” Kavi grins. “When the tens match and the ones make 10, the answer falls out in two tiny steps — faster than you can write the sum down ⚡.”
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What your child will learn
1First, check it fitsThis lightning trick only works on a special pair. Two things must be true: (1) both numbers have the same tens digit, and (2) their ones digits add up to 10. Look at 23 and 27: same tens (2 and 2) ✅, and the ones 3 + 7 = 10 ✅. Slide the bond to see the ones are partners to 10.
2The answer has two clean partsHere is the magic. For 23 × 27 the product is 621, and it splits into a front part and a back part. Build 621 and watch: the 6 hundreds are the front part, and the 21 (2 tens and 1 one) is the back part.
3The lightning rule
4Front part + back partThe two parts just slot together. The front part is really hundreds, and the back part is the last two digits — so the whole thing is one easy addition. For 23 × 27 that is 600 + 21. Tap Step → to build it.
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A peek inside
Use the lightning trick: 23 × 27.
same tens 2, ones 3 + 7 = 10 ✓ → front 2×3 = 6, back 3×7 = 21
Try this at home: Does the trick fit? Say yes or no: 23 × 27 · 34 × 36 · 23 × 28 · 45 × 55
Ready to play and learn?
Start the free interactive lesson, or print the handout for screen-free play.
Part of Math · Stage 5 · S5 on KAVI Academy — a secular, whole-child learning platform for ages 5–12.