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problem with prime numbers

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by Taniuca » Tue Aug 17, 2010 5:58 pm
what would be the easiest way to solve this type of problem?

If w is a prime number greater than 2, which of the following could be a prime number?
a) w-12
b) w^4
c) w^2
d) w/4
e) 5w
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by uwhusky » Tue Aug 17, 2010 6:10 pm
I think it has to be A) w - 12.

b) w^4 is divisible by w.
c) w^2 divisible by w as well.
d) w/4 has to be a fraction since w = prime number.
e) 5w is divisible by 5.
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by beatthegmatinsept » Tue Aug 17, 2010 8:04 pm
One more for A. Agree with uwhusky, all other choices cant be prime numbers.
You could plug in numbers to test.
eg A. 17-12 = 5 (prime number)
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by Gurpinder » Tue Aug 17, 2010 8:10 pm
Taniuca wrote:what would be the easiest way to solve this type of problem?

If w is a prime number greater than 2, which of the following could be a prime number?
a) w-12
b) w^4
c) w^2
d) w/4
e) 5w
I would go with (A).

If W is a prime number greater than 2, means that its an odd number.

a) could be because lets say W is 17. so 17-12=5
b) doesn't work because the answer would be divisible by W
c) same reason as (b)
d) the number doesn't even have to be an integer.
e) the result of this one would be also divisible by 5 or the prime number.
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by venmic » Fri Aug 20, 2010 10:10 pm
the most imporatnt concept here os

prime* prime = non prime
5*5 = 25
7*7 = 49

prime - nonprime = prime
17-12 =5

prime + prime = nonprime


more than working with numbers
work with teh concept
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by mishelk » Thu May 19, 2011 2:47 am
venmic wrote:the most imporatnt concept here os

prime* prime = non prime
5*5 = 25
7*7 = 49

prime - nonprime = prime
17-12 =5

prime + prime = nonprime


more than working with numbers
work with teh concept

"prime - nonprime = prime" Is not always true, for example,
97 - 12 = 85 (non-prime).
It could be true for certain values of w.

The other answer options would always result in a non-prime number for all prime values of w.
Thus, w - 12 is the answer.
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by GMATGuruNY » Thu May 19, 2011 3:02 am
Taniuca wrote:what would be the easiest way to solve this type of problem?

If w is a prime number greater than 2, which of the following could be a prime number?
a) w-12
b) w^4
c) w^2
d) w/4
e) 5w
Make a quick list of prime numbers greater than 2:
w = {3, 5, 7, 11, 13, 17, 19, 23, 29...}

Answer choice A works when w=29:
w-12 = 29-12 = 17, which is prime.

The correct answer is A.
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by sidvenki » Thu May 19, 2011 4:53 pm
Hi All,

Rather than CHOOSING A, we can eliminate all the others very easily.

1) w^4 and w^2 are divisible by w itself.
2) w/4, If w is a prime no, then w/4 will definitely be a fraction. So not a prime number .
3) 5w is divisible by 5.

All the other 4 options definitely will not give a prime number.

so has to yield any prime number.


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by artistocrat » Thu May 19, 2011 6:48 pm
B-E are factors or multiples. A = 17-12 or 19-12 or 23-12 etc.
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