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- multiplying by 11
- multiplying by 5 or 10/2
- multiplying by 9
- multiply by 10 then subtract the number
- sum of digits of multiples of 9 add to 9
- easy tip for 1-9 : subtract one from first digit then the second digit is 9 minus that
mar 9 2024 ∞
mar 9 2024 +
mar 9 2024 +