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I always get tripped up on multiples of 13. I wonder why that is.
by valine 11y ago
I always get tripped up on multiples of 13. I wonder why that is.
- iopq 11y agoThere is no easy way to tell multiples of 13. 91 is particularly hard because it's a multiple of 7 and 13. Obviously 5 * 13 is too easy. There's a test for division by 3, so 9 * 13 is too easy. So basically you should only miss 91.
- miracle_code 11y ago143 and 169 come to mind as well. Got 65 as highest score btw, was always a good calculator.
- madcaptenor 11y agoI've heard 91 called the "smallest non-obvious composite". Assume that testing for divisiblity by 2, 3, 5, and 11 is easy - for 2 and 5 you just look at the last digit, for 3 you take the sum of the digits, and with two-digit numbers a number is divisible by 11 if and only if both its digits are the same. Also assume that you can recognize squares. Then 91 = 7*13 is the first number that you won't recognize as composite using these test.
- sdenton4 11y agoThe more general test for eleven is to check that the alternating sum of the digits is zero.
- Bahamut 11y agoThere is a rule for divisibility by 7 - take the last digit, double it, and subtract from the rest of the numbers. If that is divisible by 7, the original number is too. For example, 91 gives 9 - 2 * 1 = 7, so it is divisible by 7
- EpicDavi 11y agoNeat trick! Just used it to find out that this post id is divisible by 7.
- sebastos 11y agoWell, I feel better for going out on 91 now after only 5 numbers.
- noobie 11y agoFor 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.
- rangibaby 11y agoAt school we did exercises to memorize from 1x1 up to 12x12, so even now I can remember all of them (and powers of two, of course :-))