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11 Oct 2007, 08:33
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N
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Can any on pls explain why the answer is C. Thanks.
P1 and P2 are both prime. Determine p1
1. p2 - p1 = 2
2. (p2)^2 - (P1)^2 = 120
P1 and P2 are both prime. Determine p1
1. p2 - p1 = 2
2. (p2)^2 - (P1)^2 = 120
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11 Oct 2007, 08:54
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Welcome to GMAT Club ! 
Yes... (C) it is
Stat1
p2-p1 = 2
o If p2 = 7 then p1 = 5
o If p2 = 5 then p1 = 3
INSUFF.
Stat2
(p2)^2 - (P1)^2 = 120
That implies,
o (p2 - p1)*(p2 + p1) = 6 * 20 => p2 = 13 and p1 = 7
or
o (p2 - p1)*(p2 + p1) = 4 * 30 => p2 = 17 and p1 = 13
or
o (p2 - p1)*(p2 + p1) = 2 * 60 => p2 = 31 and p1 = 29
INSUFF
Both 1 and 2
2*(p2+p1) = 120
<=> p2+p1 = 60
<=> p2=31 and p1=29
SUFF.
Yes... (C) it is
Stat1
p2-p1 = 2
o If p2 = 7 then p1 = 5
o If p2 = 5 then p1 = 3
INSUFF.
Stat2
(p2)^2 - (P1)^2 = 120
That implies,
o (p2 - p1)*(p2 + p1) = 6 * 20 => p2 = 13 and p1 = 7
or
o (p2 - p1)*(p2 + p1) = 4 * 30 => p2 = 17 and p1 = 13
or
o (p2 - p1)*(p2 + p1) = 2 * 60 => p2 = 31 and p1 = 29
INSUFF
Both 1 and 2
2*(p2+p1) = 120
<=> p2+p1 = 60
<=> p2=31 and p1=29
SUFF.
11 Oct 2007, 09:05
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skyjuice
1.There are many primes with difference 2, e.g. 3 and 5, 5 and 7. Therefore, 1 in INSUFF.
2. There are only two primes that satisfy this equation and differ by 2.
In fact, I was not sure whether there were more than two primes that satisfied just (2). 120 is not much, and if we try to put some numbers, we will see that difference between p1^2 and p2^2 becomes larger and larger after 29 and 31. However, then I tried 11 and 1.
It is C. 
