3 ms·
Most 2 digit numbers not divisible by 2, 3, or 5 are prime
- AnimalMuppet 3y agoLooks like there's a missing space after 91. That makes 91.49 look like a decimal, rather than the start of another sentence.
- hiAndrewQuinn 3y agoKiitos, will rm in the morning
- madcaptenor 3y agoI've heard this as 91 is the smallest (or only two-digit) composite number that "looks prime".
- hiAndrewQuinn 3y agoThat's a really good way to put it, it's the only one I might reliably trip up on.
- zgs 3y agoThis is really obvious. All two digits composite numbers must have a factor less than or equal to their square root. Square root of 100 is 10. Thus all composite numbers under 100 must have a factor under 10. There are only four primes less than 10: 2, 3, 5 and 7. Once you exclude all numbers divisible by 2, 3 and 5 these are the composite numbers left: 49, 77, 91.