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 Threes and sevens (Posted on 2006-10-25)
You can decide whether a number is divisible by 3 by forming its cross sum, that is the sum of all its decimal digits: It is divisible by three exactly if its cross sum is. Similarly, a three-digit number is divisible by seven if (and only if) the sum of twice its most significant digit plus three times its middle digit plus its least significant digit is divisible by seven.

Can you find a similar scheme for checking divisibility by 33 and 37, for numbers with an arbitrary number of digits?

 No Solution Yet Submitted by vswitchs Rating: 2.5000 (2 votes)

 Subject Author Date dumb calculator answer jduval 2007-09-16 19:30:17 re: Generally speaking ... JLo 2006-10-25 12:24:32 Another answer for 37 JLo 2006-10-25 12:19:36 Generally speaking ... Steve Herman 2006-10-25 12:15:50 Another answer for 33 JLo 2006-10-25 11:40:41 answer for 37 Dennis 2006-10-25 10:32:05 Answer for 33 Old Original Oskar! 2006-10-25 09:45:10

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