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Sometimes there are hidden patterns in numbers that you may not notice right away. One example of this can be found in a barcode, a sequence of numbers used to identify a product.
Try it yourself – find an object with a 13-digit barcode. (If you're in the US, a 12-digit barcode will work too. If you imagine an extra
0 on the front, you're using a slightly different system, so the magazine won't do that, so the book won't work. You can use a copy New Scientist. Combine 1st, 3rd, 5th digits, etc. to get the total.
Source: www.newscientist.com