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.