Comment on Amazon should've made Prime Day fall on a date corresponding to an actual three-digit prime number.

EmoDuck@sh.itjust.works ⁨2⁩ ⁨months⁩ ago

• olafurp@lemmy.world ⁨2⁩ ⁨months⁩ ago

  Divisible by 3. Easy to check since 7 + 1 + 7 = 15 which is divisible by 3.

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  • FeatherConstrictor@sh.itjust.works ⁨2⁩ ⁨months⁩ ago

    Oh awesome that’s a neat trick I’ve never seen before. How does that work? For a number like 700 for example, 7 + 0 + 0 = 7 but 700 is visible by 10.

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    • Wasgaytsiedasan@feddit.de ⁨2⁩ ⁨months⁩ ago

      You can only use this method to check if the number can be divided by 3.

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      • booly@sh.itjust.works ⁨2⁩ ⁨months⁩ ago

        It works for 9, too.

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    • derpgon@programming.dev ⁨2⁩ ⁨months⁩ ago

      It works differently for each number. For 2, the last number has to be divisible by 2. For 3, the sum of the digits has to be divisible by 3 For 5, the number has to end with a 0 or a 5. For 7, it is kinda tricky. Take the last digit, double it, and subtract it from the numbers on the left. If the remainder is 0 or divisible by 7, the whole number is divisible by 7. For example 49: 9×2=18, 4-18=-14, -14/7=2 with remainder 0. For 700, 0×2=0, 70-0=70, 70/7=10 remainder 0.

      This is usually specified for prime numbers, for non-prime number, you just do calculate the prime components of a number and combine the rules.

      For example, divisibility by 15: it has to be divisible by 3 and 5. 1+5=6, 6/3=2 remainder 0. 15 ends with a 5. For number where with multiple same prime components the rules for these duplicate numbers have to apply multiple times. Like for 25, it has to end with a 5 or 0, and when dividing the number by 5, the result has to end with a 5 or a 0 aswell.

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    • Audacious@sh.itjust.works ⁨2⁩ ⁨months⁩ ago

      Back in the olden days before digital calculators, people had tricks like that to help them do mental math.

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      • FeatherConstrictor@sh.itjust.works ⁨2⁩ ⁨months⁩ ago

        😂 I’m familiar with many of them because I liked math in school and went through all of primary and middle school without a digital calculator. This one is one I hadn’t heard of before and, again because I like math, I’m interested in understanding 1) what the trick is and 2) mathematically how it works out

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    • owsei@programming.dev ⁨2⁩ ⁨months⁩ ago

      A programmer I know wrote a small paper about this

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