3 ms·
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'
by 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 agoOf 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…