There’s also a simple trick to multiplying by 9s: The multiples of 9 have the property that their digits add up to 9 (9 × 2 = 18 and 1 + 8 = 9). Also, the first digit of the answer when multiplying by 9 is 1 less than the multiplier (e.g., 9 × 3 = 27 begins with 2).
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feasthat - (no access) - Secrets of Mental Math (guidebook).pdf, p10
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