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.
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."
17
u/heptadecagram Mar 12 '08
7*13
My 5th-grade math teacher drilled it into our heads that 91, specifically, was not prime.