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.
No it’s 51
Naw, 5 + 1 = 6, 6 is divisible by 3, so 51 is divisible by 3.
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
