BREAKING: Target Test Prep releases Brand New 2026 On Demand GMAT prep course

Redeem

sophie germain prime

This topic has expert replies
Post new topic Post Reply
Previous Topic Next Topic
Master | Next Rank: 500 Posts
Posts: 226
Joined: Thu Nov 25, 2010 12:19 am
Thanked: 3 times
Followed by:2 members

sophie germain prime

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
Top
Source: — Problem Solving |
Master | Next Rank: 500 Posts
Posts: 142
Joined: Thu Apr 26, 2012 3:24 am
Location: India
Thanked: 28 times

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"
Top
Post new topic Post Reply
• Page 1 of 1