I always found it a bit frustrating that there was no divisibility by 7 criteria. Well I just learned one!
You subtract the 2 times the unit digit to the rest and you continue until you have something small enough to see if it is a multiple of 7.
Example: 3178 gives you
317 - 16 = 301, then
30 - 2 = 28, and 28 is divisible by 7.
Thus 3178 is divisible by 7!
Apparently a 12 years old boy came up with that algorithm: https://goodnigeria.com/chika-ofili-provides-solution-to-a-mathematics-problem/
@erou
And, substituting 2 with 9 and following the same procedure is the test for divisibility by 13. Although, I got no clue if the same 12 years old came up with this as well.
#math #maths #mathematics #divisibility #primes #primenumber #primenumbers
@erou oh sorry. Thanks for clearing doubts
@Theraviranjankr @vipaal Well it does though!
250 - 9*9 = 169 and 169 is still divisible by 13.
You can go on:
16 - 9*9 = -65 and 65 is divisible by 13. It's a bit weird because it's negative, but it works!