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Question
How do you multiply a two-digit number by 11 in your head?
Answer
Split the two digits apart and drop their sum in the middle — carrying if that sum reaches 10.
Because ×11 = ×10 + ×1, the middle digit becomes the sum of the outer two:
- 25 × 11 → 2 (2+5) 5 → 275.
- 72 × 11 → 7 (9) 2 → 792.
The carry pitfall (the most mis-stated trick online): if the digit sum is 10 or more, carry the ten leftward. 85 × 11 → 8 (13) 5 → the 13 gives 3 with 1 carried: (8+1) 3 5 → 935, not 8135.
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