Multiples of 7?

Question

I recall a primary school teacher who gave me a trick to recall numbers multiples of 3 & 7. Only I've forgotten how the trick for 7 went.

It was something like the x3. Which was that adding the digets of multiples of 3 will always add to 3, 6 or 9. Eg 81 = 8+1 = 9 & 642 = 6+4+2 = 12 -> 1+2 = 3.

Does anyone know the x7 one?

Yssa A · Accepted Answer

you get the last digit of the number then multiply it by two, then subract it from the remaining digits.. that number is divisible by seven if the answer is zero or another number that is divisible by 7.

for example,

161

1 x 2 = 2 <--1 is the last digit then you multiply it to 2

then, 16-2=14 <--you subract 2 from 16 which are the remaining digits of 161

14 is already divisible by seven but you can do the whole thing again if you want to..

:)

matchbox20_is · Answer

I'm not sure if there's any trick for that, but maybe you're thinking of division by 7.

1/7 = 0.142857 142857 142857 ...etc.
2/7 = 0.285714 285714 285714 ...etc.
3/7 = 0.428571 428571 428571 ...etc.
4/7 = 0.571428 571428 571248 ...etc.
5/7 = 0.714285 714285 714285 ...etc.
6/7 = 0.857142 857142 857142 ...etc.

The next multiple of 1/7 is simply finding the next highest digit in the "142857" cluster and repeating it!

Multiples of 7?

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