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evil prime numbers...

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by schumi_gmat » Tue Sep 30, 2008 9:31 am
Ans is E

Please post OA
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by kris610 » Tue Sep 30, 2008 9:39 am
I go with B. 7 and 3. Basically, the solution should be such that the numerator is the sum of two different primes and the denominator a product of those primes.
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by schumi_gmat » Tue Sep 30, 2008 9:40 am
my bad. It is B
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by stop@800 » Tue Sep 30, 2008 9:40 am
Ans is B

primes are 3 and 7
1/3 + 1/7 = 10 / 21


for E
19 / 77
77 gives primes as 11 and 7
so sum will be 18 and not 19.

HTH
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by smallsorrow » Tue Sep 30, 2008 9:46 am
It is E!!
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by EricLien9122 » Tue Sep 30, 2008 9:54 am
The question is asking (1/x)+(1/y)=? x and y are prime.

1/x+1/y=(x+y)/(xy) (work formula).

since x and y are prime, xy can not equal a prime number and can only have 2 divisors excluding 1.

A: 7/13, 13 is prime, wrong answer.
B. 10/21, 21 is not prime and can divide by 3 and 7, possible choice.
C. 11/30, 30 can divide by 2,3,10...wrong answer.
D. 23/50, 50 can divide by 2,5,10...wrong answer.
E. 19/77, 77 can divide by 7, 11, possible choice.

E. 7+11=18, while answer choice has 19, wrong answer.
B. 3+7=10, right choice.

B is the correct answer.

Please post your thoughts and share with everyone.
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by smallsorrow » Tue Sep 30, 2008 10:03 am
Right - I just have the answer from this official gmat paper test and it states E is correct...



(If I am wrong (cause I just dont have all papers with me), I will say so later)
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by vittalgmat » Tue Sep 30, 2008 10:34 am
schumi_gmat wrote:Ans is E

Please post OA
I think the answer is B.
E says 19/77 instead of 18/77. Anyway, the explanation is like this.

Consider two different prime numbers. Say 3 and 7. Their reciprocal
is 1/3 and 1/7 respectively. The question stem says to add these.
ie. 1/3 + 1/7 => 10/21 which is B.

Choice E says 19/77. 77 is a product of two prime numbers 7 and 11.
Lets do the same drill as above.
1/7 + 1/11 => (7+11)/ (7 *11) => 18/77.

The way to approach this problem is to look at the Denominators and figure out the two prime numbers coz denominator is product of two prime #. From there on, follow the question stem to verify if the Numerator the u computed is same as the one in the choice.

HTHelps
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