4 ms·
For me it's 17, 17*3= 51 which at a first quick glance looks like a prime, up until you add the digits and it's clear of course.
by noobie 11y ago
For me it's 17, 17*3= 51 which at a first quick glance looks like a prime, up until you add the digits and it's clear of course.
- jemfinch 11y agoUse the test for divisibility by 3: if the sum of the digits is divisible by 3, the number is divisible by 3. 5+1=6, so 51 is divisible by 3.
- noobie 11y agoOh yeah, that's what I meant by saying "up until you add the digits and it's clear of course.". It's just an instant observation I guess.
- borkabrak 11y agoThat's one where experience playing darts really helps. :-)
- notJim 11y agoWhere I'm from, 17 and 51 are two freeways that run kinda parallel to each other, so that was an easy one for me.