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Problem
Multiplying two-digit numbers by 11 using traditional methods takes time.
Solution
Add the two digits and place the sum between them. If sum > 9, carry the 1.
Benefit
Reduces calculation time by 80% and can be done mentally.
Example
23 × 11:
2 _ 3 → 2 (2+3) 3 → 253
67 × 11:
6 _ 7 → 6 (6+7) 7 → 6(13)7 → 737
89 × 11:
8 _ 9 → 8 (8+9) 9 → 8(17)9 → 979