Enigma 1561: Some check!
22 February 2012
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From New Scientist #2724, 5th September 2009 [link]
Joe has been teaching Penny all about “checksums”, which have been used for years to check the accuracy of calculations.
To calculate the checksum of a number you add up its digits. If that total is a single digit then that is the checksum; if not, the digits in the total are added to produce a new total and the process is repeated until a single-digit answer is produced. That single digit is the original number’s checksum.
What is the checksum of the number (2100 + 3200)300?