ISBN numbers & the number 11

“A modern though little realised example of undecimal counting is seen in the ISBN of published books. Any ISBN comprises ten digits. If you multiply the first by ten, the second by nine, the third by eight, and so on, summing the results as you go along, the result will always be divisible by eleven.”

What Are The Chances Of That?: Fabulous Facts About Figures
by William Hartston

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