If p is a positive integer, is 2p + 1 prime?

This topic has expert replies
Moderator
Posts: 7187
Joined: Thu Sep 07, 2017 4:43 pm
Followed by:23 members

Timer

00:00

Your Answer

A

B

C

D

E

Global Stats

If p is a positive integer, is 2p + 1 prime?

(1) p is prime

(2) units digit of p is not prime

OA E

Source: Official Guide

User avatar
GMAT Instructor
Posts: 15539
Joined: Tue May 25, 2010 12:04 pm
Location: New York, NY
Thanked: 13060 times
Followed by:1906 members
GMAT Score:790

by GMATGuruNY » Sat Mar 09, 2019 3:14 am

Timer

00:00

Your Answer

A

B

C

D

E

Global Stats

BTGmoderatorDC wrote:If p is a positive integer, is 2p + 1 prime?

(1) p is prime

(2) units digit of p is not prime

OA E

Source: Official Guide
Statements combined:
Case 1: p=11 --> p is prime, but its units digits is not prime
In this case, 2p+1 = 2*11 + 1 = 23.
Since 23 is prime, the answer to the question stem is YES.

Case 2: p=19 --> p is prime, but its units digit is not prime
In this case, 2p+1 = 2*19 + 1 = 39.
Since 39 is not prime, the answer to the question stem is NO.

Since the answer is YES in Case 1 but NO in Case 2, the two statements combined are INSUFFICIENT.

The correct answer is E.
Private tutor exclusively for the GMAT and GRE, with over 20 years of experience.
Followed here and elsewhere by over 1900 test-takers.
I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia -- a long list of countries.
My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua -- a long list of top programs.

As a tutor, I don't simply teach you how I would approach problems.
I unlock the best way for YOU to solve problems.

For more information, please email me (Mitch Hunt) at [email protected].
Student Review #1
Student Review #2
Student Review #3