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sophie germain prime

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by nafiul9090 » Tue May 15, 2012 8:44 pm
A "Sophie Germain" prime is any positive prime number p for which 2p + 1 is also prime. The product of all the possible units digits of Sophie Germain primes greater than 5 is

(A) 3

(B) 7

(C) 21

(D) 27

(E) 189

is there any short cut method??

oa is D
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by mathbyvemuri » Tue May 15, 2012 10:16 pm
Generally a prime ends with 1,3,7 or 9. But not all numbers with these digits as unit digits are prime.
Out of these, "Sophie Germain" prime's unit digit takes only 1,3 or 9. A unit digit 7 is not qualified because for that, 2p+1 yields a unit-digit of 5, which does not result in a prime.
The possible unit digits for "Sophie Germain" prime are: 1,3 and 9
The product of all possible unit digits = [spoiler]1*3*9 = 27 [/spoiler]
Answer "D"
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