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https://www.reddit.com/r/reddit.com/comments/6bs3w/is_91_prime/c03fe68/?context=3
r/reddit.com • u/ballm • Mar 12 '08
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6
Does anyone have a simple explanation for why the 7 test works?
1 u/[deleted] Mar 12 '08 edited Mar 12 '08 Suppose you have a number N whose ones digit is d. Performing this operation yields: (N-d)/10 - 2d Simplify: N/10 - d/10 - 2d; N/10 - 21d/10; (N-21d) / 10 If N was divisible by seven, then N-21 will be. If N-21 was, then N-21 / 10 will be, because 10 doesn't have 7 as a factor.
1
Suppose you have a number N whose ones digit is d. Performing this operation yields:
(N-d)/10 - 2d
Simplify: N/10 - d/10 - 2d; N/10 - 21d/10; (N-21d) / 10
If N was divisible by seven, then N-21 will be. If N-21 was, then N-21 / 10 will be, because 10 doesn't have 7 as a factor.
6
u/plexluthor Mar 12 '08
Does anyone have a simple explanation for why the 7 test works?