Math · Stage 6
Multiply by 99 and 999 by rounding and subtracting
99 is just one less than 100, and 999 one less than 1000. So multiply by the round number first, then give back one of the number — no long multiplication needed.
✦ Math📘 Stage 6 · S6⏱ ~12 min🧒 Ages 10–12★ Free to start
I can multiply by 99 and by 999 using the round-number-minus-itself shortcut.
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Kavi's 99-Coin Night Market ⚡
Every lantern at the market costs 99 coins. A customer buys 7 of them! Kavi never multiplies 7 × 99 the long way. "Charge for 7 whole hundreds, then hand 7 coins back," she hoots — 700 − 7 = 693. Because 99 is only one less than 100, every lantern is 1 coin cheaper, so Kavi gives back exactly one coin per lantern.
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What your child will learn
1× 99 = × 100, then give the number back99 is one less than 100. So to multiply by 99, first multiply by 100 (easy — just slide two places left), then subtract the number itself once, because every group of 99 is 1 short of a hundred. Tap a row.
2× 999 = × 1000, then give the number back999 is one less than 1000. Same idea, one column further: multiply by 1000 (slide three places left), then subtract the number once. Each group of 999 is 1 short of a thousand. Tap a row.
3The second step is just a subtractionThe × 100 part is instant. The real work is the take-away. Here is 700 − 7 — the last step of 7 × 99. Tap Step → to borrow across the zeros and land on 693.
4Check it in one glanceA × 99 answer is always a little less than the round hundred, and a × 999 answer a little less than the round thousand. If someone lands exactly on the round number, they forgot to subtract. Tap a row.
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A peek inside
Multiply 7 × 99 with the shortcut.
× 99 is × 100 then subtract the number: 7 × 100 = 700, then 700 − 7
Try this at home: × 99 the quick way: 3 × 99 = __ · 8 × 99 = __ · 5 × 99 = __
Ready to play and learn?
Start the free interactive lesson, or print the handout for screen-free play.
Part of Math · Stage 6 · S6 on KAVI Academy — a secular, whole-child learning platform for ages 5–12.