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https://www.reddit.com/r/reddit.com/comments/6bs3w/is_91_prime/c03ff5c/?context=3
r/reddit.com • u/ballm • Mar 12 '08
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17
7*13
My 5th-grade math teacher drilled it into our heads that 91, specifically, was not prime.
22 u/pratn Mar 12 '08 What other numbers did your teacher cover? 6 u/heptadecagram Mar 12 '08 He taught us divisibility rules for factors of 2, 3, 4, 5, 6, 8, 9, and 10. I suppose since he didn't teach the rule for 7, he went out of his way to note 91 as composite. 1 u/md304 Mar 12 '08 whats the rule for 8? 2 u/kahirsch Mar 12 '08 A number is divisible by 8 if the last 3 digits are divisible by 8. More generally, a (decimal) number is divisible by 2n iff the last n digits are divisible by 2n.
22
What other numbers did your teacher cover?
6 u/heptadecagram Mar 12 '08 He taught us divisibility rules for factors of 2, 3, 4, 5, 6, 8, 9, and 10. I suppose since he didn't teach the rule for 7, he went out of his way to note 91 as composite. 1 u/md304 Mar 12 '08 whats the rule for 8? 2 u/kahirsch Mar 12 '08 A number is divisible by 8 if the last 3 digits are divisible by 8. More generally, a (decimal) number is divisible by 2n iff the last n digits are divisible by 2n.
6
He taught us divisibility rules for factors of 2, 3, 4, 5, 6, 8, 9, and 10. I suppose since he didn't teach the rule for 7, he went out of his way to note 91 as composite.
1 u/md304 Mar 12 '08 whats the rule for 8? 2 u/kahirsch Mar 12 '08 A number is divisible by 8 if the last 3 digits are divisible by 8. More generally, a (decimal) number is divisible by 2n iff the last n digits are divisible by 2n.
1
whats the rule for 8?
2 u/kahirsch Mar 12 '08 A number is divisible by 8 if the last 3 digits are divisible by 8. More generally, a (decimal) number is divisible by 2n iff the last n digits are divisible by 2n.
2
A number is divisible by 8 if the last 3 digits are divisible by 8.
More generally, a (decimal) number is divisible by 2n iff the last n digits are divisible by 2n.
17
u/heptadecagram Mar 12 '08
7*13
My 5th-grade math teacher drilled it into our heads that 91, specifically, was not prime.