m4m4m4m4@lemmy.world to Science Memes@mander.xyzEnglish · 16 hours agoAnd 299999999 is divisible by 13lemmy.worldimagemessage-square4fedilinkarrow-up1287arrow-down16
arrow-up1281arrow-down1imageAnd 299999999 is divisible by 13lemmy.worldm4m4m4m4@lemmy.world to Science Memes@mander.xyzEnglish · 16 hours agomessage-square4fedilink
minus-squareTwilightKiddy@programming.devlinkfedilinkEnglisharrow-up21·14 hours agoThe divisability rule for 7 is that the difference of doubled last digit of a number and the remaining part of that number is divisible by 7. E.g. 299’999 → 29’999 - 18 = 29’981 → 2’998 - 2 = 2’996 → 299 - 12 = 287 → 28 - 14 = 14 → 14 mod 7 = 0. It’s a very nasty divisibility rule. The one for 13 works in the same way, but instead of multiplying by 2, you multiply by 4. There are actually a couple of well-known rules for that, but these are the easiest to remember IMO.
The divisability rule for 7 is that the difference of doubled last digit of a number and the remaining part of that number is divisible by 7.
E.g. 299’999 → 29’999 - 18 = 29’981 → 2’998 - 2 = 2’996 → 299 - 12 = 287 → 28 - 14 = 14 → 14 mod 7 = 0.
It’s a very nasty divisibility rule. The one for 13 works in the same way, but instead of multiplying by 2, you multiply by 4. There are actually a couple of well-known rules for that, but these are the easiest to remember IMO.