7 ms·

I agree. And I would add 51 to that.

• TheDong 5y ago
  51's an easy one because 5+1=6, and 6 is divisible by 3, so it must be divisible by 3. It's easy to try 'divisible by 5' (ends in 5 or 0) and 'divisible by 3' (sum of digits is divisible by 3). 91 isn't found as prime by those two tests, so it needs the extra exceptional rule.
  • bbx 5y ago
    Of course! I actually know of this divisible by 3 rule, and use it to test out all numbers, like 87, 111, or 69, but somehow forget to apply it to 51…
• Tempest1981 5y ago
  They have stats on the most common mistakes here: https://isthisprime.com/game/record.php https://isthisprime.com/game/record.php (loads very slowly) 51 is #1, then 57, then 1
  • nilstycho 5y ago
    It’s very surprising to me that 91 isn’t in the top 20. It’s beaten by 77 and 21?