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Check Codes

Details are given of how check codes are constructed (using modulus arithmetic for passports, bank accounts, credit cards, ISBN book numbers, and so on. A list of codes is given and you have to check if they are valid identification numbers?

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Check Code Sensitivity

You are given the method used for assigning certain check codes and you have to find out if an error in a single digit can be identified.

Transposition Fix

Age 14 to 16 Challenge Level:
Look for a case where it fails and if you can't find one try to find a general proof.