Prime number problem

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nafishasan: Junior | Next Rank: 30 Posts; Posts: 27; Joined: Thu Mar 17, 2011 11:02 pm

by nafishasan » Sun Feb 05, 2012 10:33 pm

P is a prime number. If 6p is between 15 and 95, which of the following can be a value of 7p + 2 ?

A. 15
B. 79
C. 121
D. 65
E. 21

Last edited by nafishasan on Sun Feb 05, 2012 11:01 pm, edited 1 time in total.

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Source: Beat The GMAT — Problem Solving | Sun Feb 05, 2012 10:33 pm

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Anurag@Gurome: GMAT Instructor; Posts: 3835; Joined: Fri Apr 02, 2010 10:00 pmFollowed by: 523 members; Location: Milpitas, CA; Thanked: 1854 times; GMAT Score:770

by Anurag@Gurome » Sun Feb 05, 2012 10:37 pm

nafishasan wrote:P is a prime number. If 6p is between 15 and 95, which of the following can be a value of ?

A. 15
B. 79
C. 121
D. 65
E. 21

This seems to be incomplete, can you please check this again?

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Anurag@Gurome: GMAT Instructor; Posts: 3835; Joined: Fri Apr 02, 2010 10:00 pmFollowed by: 523 members; Location: Milpitas, CA; Thanked: 1854 times; GMAT Score:770

by Anurag@Gurome » Sun Feb 05, 2012 10:47 pm

nafishasan wrote:P is a prime number. If 6p is between 15 and 95, which of the following can be a value of 7p + 2?

A. 15
B. 79
C. 121
D. 65
E. 21

The missing term is 7p + 2.

If p = 3 (the minimum possible value of p), then 6p = 18, which is between 15 and 95.
So, 7p + 2 = 7 * 3 + 1 = 22, none of the answer choice.
The maximum possible value of p = 13 (as 6 * 17 = 102, out of range) then 6p = 6 * 13 = 78. So, 7p + 2 = 7 * 13 + 2 = 93, none of the answer choice.
Try other possible prime numbers less than 13. If p = 11, then 7p + 2 = 79, which is option B.

The correct answer is B.

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LalaB: Master | Next Rank: 500 Posts; Posts: 425; Joined: Wed Dec 08, 2010 9:00 amFollowed by: 7 members; Thanked: 56 times; GMAT Score:690

by LalaB » Mon Feb 06, 2012 9:34 am

7p+2=79
p=11 . P is a prime number, 6p is between 15 and 95

B is correct

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