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# What is the quick litmus test to see whether an integer is

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CEO
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What is the quick litmus test to see whether an integer is [#permalink]

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26 Oct 2007, 09:48
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What is the quick litmus test to see whether an integer is divisible by 7
VP
Joined: 10 Jun 2007
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26 Oct 2007, 12:37
bmwhype2 wrote:
What is the quick litmus test to see whether an integer is divisible by 7

No rules that I know of.
Prime factorization? Or just divide by 7, shouldn't take that long...
CEO
Joined: 21 Jan 2007
Posts: 2741
Location: New York City
Followers: 11

Kudos [?]: 942 [0], given: 4

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26 Oct 2007, 12:39
bkk145 wrote:
bmwhype2 wrote:
What is the quick litmus test to see whether an integer is divisible by 7

No rules that I know of.
Prime factorization? Or just divide by 7, shouldn't take that long...

I came across several questions that asked this concept.

is 342234234123 divisible by 7? lol
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26 Oct 2007, 12:50
bmwhype2 wrote:
bkk145 wrote:
bmwhype2 wrote:
What is the quick litmus test to see whether an integer is divisible by 7

No rules that I know of.
Prime factorization? Or just divide by 7, shouldn't take that long...

I came across several questions that asked this concept.

is 342,234,234,123 divisible by 7? lol

I use approximation

since ---> 7*4 = 28 ---> then:

342,234,234,123 - 280,000,000,000 = 62,234,234,123

7*8 = 56

62,234,234,123 - 56,000,000,000 = 6,234,234,123

6,234,234,123 - 5,600,000,000 = 634,234,123

634,234,123 - 560,000,000 = 74,234,123

7*10 = 70

74,234,123 - 70,000,000 = 4,234,123

7*6 = 42

4,234,123 - 4,200,000 = 34,123

34,123 - 28,000 = 6,123

6,123 - 5,600 = 523

523 - 490 = 33

this number dosen't divide by 7 !

VP
Joined: 10 Jun 2007
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26 Oct 2007, 13:19
bmwhype2 wrote:
bkk145 wrote:
bmwhype2 wrote:
What is the quick litmus test to see whether an integer is divisible by 7

No rules that I know of.
Prime factorization? Or just divide by 7, shouldn't take that long...

I came across several questions that asked this concept.

is 342234234123 divisible by 7? lol

I would just divide that number by 7. Should take about 20 seconds max.
Do you have an actual problem?
Re: Divisibility Rule   [#permalink] 26 Oct 2007, 13:19
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