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
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
No it’s 51
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 is not divisible by 3!
It is divisible by 3
Actually 51/3! Is 8.5
You bastard
Agreed. 51 looks prime AF.
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.
Surely it’s 1? Looks prime, and some people will even argue that it is!
Its only factors are 1 and itself after all
