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Composite primes

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Source: — Data Sufficiency |
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by cramya » Sun Dec 07, 2008 12:47 pm
Take the easier of the 2 statements:

Stmt II

2 digit number greater than 20 with tens digit 2


23 - not composite
24 - composite

INSUFF

Stmt I

Numbers

22,24,26,28,30,33,36,39,40,,44,48,50,55,60,66,70,77,80,88,90,99

All of these are composite

SUFF

A)

Other than listing the numbers which can be done within 2 minutes I coudnt think of a shorcut
Last edited by cramya on Sun Dec 07, 2008 12:50 pm, edited 1 time in total.
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by logitech » Sun Dec 07, 2008 12:49 pm
cramya wrote: Other than listing the numbers which can be done within 2 minutes I coudnt think of a shorcut
You don't need a shortcut here bro. Good thinking , clean solution! Sweet!
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by cramya » Sun Dec 07, 2008 12:51 pm
Keep in mind the following:

1) 0 is a multiple of every number
2) 1 is a factor of every number
3) 0 has infinetely many factors (thats why I added 30,40,50,60,... to the list above since the tens digit in each number is a factor of units digit)
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by pbanavara » Mon Dec 08, 2008 7:07 pm
From 1: Since the Tens has to be a factor of the Units - The number has to have a factor which is not 1 and itself. Hence it can't be prime. Trial and error - but took less than 2 mins

- pradeep
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by earth@work » Mon Dec 08, 2008 8:24 pm
cramya wrote:Keep in mind the following:

1) 0 is a multiple of every number
2) 1 is a factor of every number
3) 0 has infinetely many factors (thats why I added 30,40,50,60,... to the list above since the tens digit in each number is a factor of units digit)
cool! "1 is a factor of every number" just slipped my mind while solving this problem.
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by vishubn » Mon Dec 08, 2008 9:31 pm
Cramya: 23 is not composite !! but arent we supposed t oconsider only numbers which are composite ! and wit hthis how can u proce D is Insuff!

I am sure i am askign somethign simple ! but i dont get it !! sorry

VIshu
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by cramya » Mon Dec 08, 2008 9:34 pm
Cramya: 23 is not composite !! but arent we supposed t oconsider only numbers which are composite ! and wit hthis how can u proce D is Insuff!

I am sure i am askign somethign simple ! but i dont get it !! sorry
No probs Vishu!

U r correct. . 23 is prime and not composite.

Stmt II says 2 digit number greater than 20 with tens digit of 2

If its 23 then its prime
If its 24 then its composite

INSUFF since 2 different answers (hence D) not possible)


In the problem they have given the definition of a composite number which is a bonus and they are not telling us that we need to consider only composites.

Hope this helps!
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by vishubn » Mon Dec 08, 2008 9:38 pm
In the problem they have given the definition of a composite number which is a bonus and they are not telling us that we need to consider only composites.
More than the solution , the statement helped :) thanks cramya ! i am sure i am all konked :)

thanks again

Vishu
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