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To Make Any Number Divisible By Nine By Adding A Figure To It

If (for example) the number named be 72,857, you tell the person who

names it to place the number 7 between any two figures of that sum,

and it will be divisible by 9; for if any number be multiplied by 9,

the sum of the figures of the product will be either 9, or a number

divisible by 9.