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Is integer r prime?

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by VJesus12 » Fri Apr 27, 2018 12:46 am

Timer

00:00

Answers

A

B

C

D

E

Stats

Difficulty—

Is integer r prime?

(1) r + 1 is prime.
(2) r is greater than 10.

The OAis the option C.

How can I, using both statements together, get an answer here? I need some help.
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Source: — Data Sufficiency |
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by GMATinsight » Fri Apr 27, 2018 12:58 am
VJesus12 wrote:Is integer r prime?

(1) r + 1 is prime.
(2) r is greater than 10.

The OAis the option C.

How can I, using both statements together, get an answer here? I need some help.
Question: Is Integer r Prime?

Statement 1: r+1 is Prime

i.e. r may be 2 or r may be 4 hence

NOT SUFFICIENT

Statement 2: r is greater than 10/b]

i.e. r may be 11 or 12 hence

NOT SUFFICIENT

Combining the two statements

r is greater than 10 and r+1 is prime i.e. r can NEVER be Prime as there is only one pair of two consecutive numbers that are prime (2 and 3)

SUFFICIENT

Answer: option C
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by Vincen » Fri Apr 27, 2018 3:09 am
VJesus12 wrote:Is integer r prime?

(1) r + 1 is prime.
(2) r is greater than 10.

The OAis the option C.

How can I, using both statements together, get an answer here? I need some help.
(1) r + 1 is prime.

This tells us that r is even. Then r can be 2 (prime), 4 (not prime), 6 (not prime), 10 (not prime), ... NOT SUFFICIENT.

(2) r is greater than 10.

Since r is greater that 10, then r can be 11, 12, 13, 14, ... NOT SUFFICIENT.

(1) r + 1 is prime. + (2) r is greater than 10.

This tells us that r is an even number greater than 10. Therefore, r cannot be EVEN. SUFFICIENT.

Hence, the correct answer is the option C
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by Jeff@TargetTestPrep » Tue May 01, 2018 9:25 am
VJesus12 wrote:Is integer r prime?

(1) r + 1 is prime.
(2) r is greater than 10.
We need to determine whether r is prime.

Statement One Alone:

r + 1 is prime.

Statement one alone is not sufficient to answer the question. For example,

If r = 6, then the answer is no. If r = 2, then the answer is yes.

Statement Two Alone:

r is greater than 10.

Statement two alone is not sufficient to answer the question. For example,

If r = 12, then the answer is no. If r = 11, then the answer is yes.

Statements One and Two Together:

From statements one and two, we see that r is greater than 10 and r + 1 is prime. Since all prime numbers greater than 2 are odd, we see that r + 1 must be odd. In that case, r must be even. However, since r > 10, it can't be a prime. So the answer is no.

Answer: C

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