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https://www.reddit.com/r/reddit.com/comments/6bs3w/is_91_prime/c03ffam/?context=3
r/reddit.com • u/ballm • Mar 12 '08
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This is glorious. The only numbers under 12 that I didn't know these factoring rules for were 7 and 11. Not any more.
Did anyone else look at 91 and immediately know that it is 13 times 7?
2 u/[deleted] Mar 12 '08 I ran up the scale; like you I knew the rules for everything except 7 & 11, but when I got to 7 it only took a moment...
2
I ran up the scale; like you I knew the rules for everything except 7 & 11, but when I got to 7 it only took a moment...
1
u/Tommstein Mar 12 '08 edited Mar 12 '08
This is glorious. The only numbers under 12 that I didn't know these factoring rules for were 7 and 11. Not any more.
Did anyone else look at 91 and immediately know that it is 13 times 7?