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Threes and sevens (Posted on 2006-10-25) Difficulty: 3 of 5
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)

Comments: ( You must be logged in to post comments.)
  Subject Author Date
dumb calculator answerjduval2007-09-16 19:30:17
re: Generally speaking ...JLo2006-10-25 12:24:32
SolutionAnother answer for 37JLo2006-10-25 12:19:36
SolutionGenerally speaking ...Steve Herman2006-10-25 12:15:50
SolutionAnother answer for 33JLo2006-10-25 11:40:41
Some Thoughtsanswer for 37Dennis2006-10-25 10:32:05
SolutionAnswer for 33Old Original Oskar!2006-10-25 09:45:10
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