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Prime Number DS

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Source: — Data Sufficiency |
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Re: Prime Number DS

by logitech » Sat Nov 08, 2008 9:40 am
jnellaz wrote:Is interger A a prime number?

1. 2A has exactly 3 factors.

2. A is an even number

Answer is A[spoiler][/spoiler]
Remember that bustard 2 is both even and prime! :P
LGTCH
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by cramya » Sat Nov 08, 2008 1:45 pm
Is interger A a prime number?

1. 2A has exactly 3 factors.

2. A is an even number

Take the easier(strictly my opinion) of the 2 statements

Stmt II A is a even number

2 is even and prime (only even prime)
4 is even and not prime

INSUFF

Down to choices A C E

Stmt II

2A has exactly 3 factors.

Juts like primes have only 2 factors(1 and itself); only squares of primes(like 4 = 2 ^ 2, 9 = 3 ^ 2) have exactly 3 factors (distinct)

When u multiply something by 2 that product is EVEN (comes in handy in lot of problems i.e may not be given when u see something multiplied by 2 we must immediately leep this in mind as it may be helpful)

The only square of a prime that is even and has 3 factors is 4 so A=2 and is Prime

SUFF

A)


Hope I have not missed something here! Good luck and let me know if u have questions.
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