The product of 11 and any 2-digit number begins and ends with the two digits of the multiplier; the number in the middle is the sum of the original two digits. Example: 23 × 11 => 2 + 3 = 5; answer: 253. For a multiplier whose digits sum to a number greater than 9, you have to carry. Example: 85 × 11 => 8 + 5 = 13; carry the 1 from 13 to the 8; answer: 935.
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feasthat - (no access) - Secrets of Mental Math (guidebook).pdf, p11
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