Some superstitious nerdy teenagers convinced small child me that 91 was a universal anomaly bc it’s 13 (bad luck) * 7 (good luck), so my weird brain never saw 91 as prime , & I can’t be the only person this happened to

No it’s 51

Naw, 5 + 1 = 6, 6 is divisible by 3, so 51 is divisible by 3.

I shared this at work recently and nobody cared. 3 6 and 9 are all kinda special.

Only if you weren’t taught (or were, but too long ago lol) the divisible by 3 rule - if the sum of the digits is divisible by 3, so is the whole number. 5+1=6 therefore 51 is divisible by 3! Bonus division rule - if it was also an even number and therefore divisible by 2, you could deduce that the whole number was divisible by 6.

Actually 51/3! Is 8.5

You bastard

Actually, 51 is not divisible by 3!

It

*is*divisible by 3Hilarious.

Surely it’s 1? Looks prime, and some people will even argue that it is!

Its only factors are 1 and itself after all

Agreed. 51 looks prime AF.

I always felt that 21 should be, but nooo. Stupid 7

21 should be

2 + 1 = 3, therefore 21 is divisible by 3 and is not prime. QED :)

9+1 = 10, therefore 91 is divisible by 10 and is not a prime … QED, I guess

In the right base, that might work.

It’s funny, but actually a good proof

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