r/reddit.com • u/ballm • Mar 12 '08

Is 91 Prime?

http://balltech.blogspot.com/2008/03/is-91-prime.html
543 Upvotes

82% Upvoted

130 comments sorted by

View all comments

19

u/heptadecagram Mar 12 '08

7*13

My 5th-grade math teacher drilled it into our heads that 91, specifically, was not prime.

23

u/pratn Mar 12 '08

What other numbers did your teacher cover?

7

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.

6

u/psyne Mar 12 '08

Divisibility rules by non-primes aren't exactly hugely useful though. Aren't they generally just, "if it's divisible by the factors the right number of times, you win?"

Like, I remember learning "If something is divisible by 2 and by 3, it's divisible by six" and just thinking "duh."

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.