Why not add the digits? If the sum of digits is divisible by 3, the number is divisible by 3. 5 + 1 is divisible by 3, so it’s not prime.
49 “looks” more prime to me because it fails that test, and if I didn’t know it’s 72, I’d say it’s probably prime.
I think the posts along these lines are already two steps too far away from the vibes.
Nobody is even considering the number being prime, it just looks like there can’t be a round number of one against the other because one is big and ends in seven and the other is relatively small and ends in 1.
If you’re even thinking about divisibility rules you’re doing it wrong. As in, your brain is too impacted by maths to see what the numbers look like from instinct alone. There’s no thinking in this, they just look weird together.
I’ve always loved math, so seeing patterns is intuitive. 3 also happens to be my favorite number, so I like looking for it in the wild.
If someone says “51 is divisible by 17,” my instinct is that it tracks because it’s obviously divisible by 3, so the other factor would be a little less than 20. That’s literally my first instinct.
7 is a weird multiple, but 3 is really easy. I guess I’m weird…
