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Bunuel can you expand on rule for a number divisible by 7 [#permalink]

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17 Apr 2014, 19:27

Hey Bunuel,

In your Math Guide for a number divisible by 7 you give this trick:

Take the last digit, double it, and subtract it from the rest of the number, if the answer is divisible by 7 (including 0), then the number is divisible by 7.

Can you give me an example on how to use it? I tried with 21 but got stuck as I couldn't interpret it properly.

I took 21 as an example and did 1 X 2 to get 2. So now do i subtract 2 from 21 or 20? or just 2? In short when you say "from rest of the number" what does that mean?

Thanks for your wonderful guide and work on this forum

Re: Bunuel can you expand on rule for a number divisible by 7 [#permalink]

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18 Apr 2014, 02:39

1

This post received KUDOS

Expert's post

patelsid wrote:

Hey Bunuel,

In your Math Guide for a number divisible by 7 you give this trick:

Take the last digit, double it, and subtract it from the rest of the number, if the answer is divisible by 7 (including 0), then the number is divisible by 7.

Can you give me an example on how to use it? I tried with 21 but got stuck as I couldn't interpret it properly.

I took 21 as an example and did 1 X 2 to get 2. So now do i subtract 2 from 21 or 20? or just 2? In short when you say "from rest of the number" what does that mean?

Thanks for your wonderful guide and work on this forum

Take the last digit, double it, and subtract it from the rest of the number, if the answer is divisible by 7 (including 0), then the number is divisible by 7.

For example, let's check whether 1519 is divisible by 7: 151-2*9=133. Since 133 is divisible by 7 (133=7*19), then so is 1,519.

For 21: 2-2*1=0. Since 0 is divisible by 7, then so is 21.

Re: Bunuel can you expand on rule for a number divisible by 7 [#permalink]

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18 Apr 2014, 07:00

@patelsid

The divisibility rule for the number 7 is of no use as far as the GMAT is concerned. It has never appeared in any form on the GMAT so far, and I don't anticipate seeing it on the exam. The divisibility rules for 2, 3, 4, 5, 6, 8, and 9 are definitely tested on the exam. (8 I haven't seen so far, but 4 I have).

Re: Bunuel can you expand on rule for a number divisible by 7 [#permalink]

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20 Apr 2014, 15:51

dabral wrote:

@patelsid

The divisibility rule for the number 7 is of no use as far as the GMAT is concerned. It has never appeared in any form on the GMAT so far, and I don't anticipate seeing it on the exam. The divisibility rules for 2, 3, 4, 5, 6, 8, and 9 are definitely tested on the exam. (8 I haven't seen so far, but 4 I have).

Cheers, Dabral

Thanks for that Dabrall, honestly it was something that was bothering me for a while since MGMAT guide also didn't have a divisibility rule for 7. So just wanted to know in general, but the fact that it is almost never tested on GMAT is great as well :D. Thanks

Re: Bunuel can you expand on rule for a number divisible by 7 [#permalink]

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20 Apr 2014, 15:52

Thanks Bunuel! That was a great example and now I can follow it easily. Once again, thanks for the time you take for the forum

Bunuel wrote:

patelsid wrote:

Hey Bunuel,

In your Math Guide for a number divisible by 7 you give this trick:

Take the last digit, double it, and subtract it from the rest of the number, if the answer is divisible by 7 (including 0), then the number is divisible by 7.

Can you give me an example on how to use it? I tried with 21 but got stuck as I couldn't interpret it properly.

I took 21 as an example and did 1 X 2 to get 2. So now do i subtract 2 from 21 or 20? or just 2? In short when you say "from rest of the number" what does that mean?

Thanks for your wonderful guide and work on this forum

Take the last digit, double it, and subtract it from the rest of the number, if the answer is divisible by 7 (including 0), then the number is divisible by 7.

For example, let's check whether 1519 is divisible by 7: 151-2*9=133. Since 133 is divisible by 7 (133=7*19), then so is 1,519.

For 21: 2-2*1=0. Since 0 is divisible by 7, then so is 21.

Hope it's clear.

gmatclubot

Re: Bunuel can you expand on rule for a number divisible by 7
[#permalink]
20 Apr 2014, 15:52

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