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Multiply by 11

90 seconds of lightning multiplication

90.0s

Technique: Multiply any number by 11

Step 1: Write the ones digit of the number as the ones digit of your answer.

Step 2: Add each pair of neighbouring digits — this gives the middle digit(s).

Step 3: Write the leading digit as-is (plus any carry).

2-digit examples (no carry)

  • 32 × 11 → Write 2, then 3+2=5, then 3. Answer: 352
  • 81 × 11 → Write 1, then 8+1=9, then 8. Answer: 891
  • 45 × 11 → Write 5, then 4+5=9, then 4. Answer: 495

When a pair sums to 10+ (carry the 1)

  • 85 × 11 → Write 5, then 8+5=13 → write 3, carry 1. Leading digit 8+1=9. Answer: 935
  • 77 × 11 → Write 7, then 7+7=14 → write 4, carry 1. Leading digit 7+1=8. Answer: 847

3-digit numbers — same pattern

  • 234 × 11 → 4, 3+4=7, 2+3=5, 2. Answer: 2574
  • 471 × 11 → 1, 7+1=8, 4+7=11 → write 1 carry 1, 4+1=5. Answer: 5181

Every question has a 30 second shot clock.

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