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

Redeem

Prime Number

This topic has expert replies
Post new topic Post Reply
Previous Topic Next Topic
Source: — Data Sufficiency |

GMAT/MBA Expert

User avatar
GMAT Instructor
Posts: 1179
Joined: Sun Apr 11, 2010 9:07 pm
Location: Milpitas, CA
Thanked: 447 times
Followed by:88 members

by Rahul@gurome » Fri Nov 19, 2010 12:56 pm
Viper83 wrote:Is Root of x a prime number?

(i) Abs (3x-7)= 2x+2

(ii) x to the power 2= 9x
Statement 1: |3x - 7| = (2x + 2)
Absolute value gives rise to two situations,
  • (1) For 3x > 7, (3x - 7) = (2x + 2) => x = 9 => √x = 3 -> Prime
    or
    (2) For 3x < 7, (7 - 3x) = (2x + 2) => x = 1 => √x = 1 -> Not Prime
Not sufficient.

Statement 2: x² = 9x
=> x² - 9x = 0 => x(x - 9) = 0 => x = 0 or 9

Not sufficient.

1 & 2 Together: x = 9 => √x = 3 -> Prime

Sufficient.

The correct answer is C.
Rahul Lakhani
Quant Expert
Gurome, Inc.
https://www.GuroMe.com
On MBA sabbatical (at ISB) for 2011-12 - will stay active as time permits
1-800-566-4043 (USA)
+91-99201 32411 (India)
Top
User avatar
Senior | Next Rank: 100 Posts
Posts: 67
Joined: Mon Aug 16, 2010 4:42 am

by Viper83 » Fri Nov 19, 2010 1:06 pm
Hi Rahul
thanks very much for your reply and explanation. In fact I solved the questions exactly like that.

My concern with this question is the following: why is the square root of 9 equal to ONLY 3. The square root of 9 should be 3 and -3. Furthermore, -3 is not a prime number.

What am I missing here?

Thanks very much!
Top

GMAT/MBA Expert

User avatar
GMAT Instructor
Posts: 1179
Joined: Sun Apr 11, 2010 9:07 pm
Location: Milpitas, CA
Thanked: 447 times
Followed by:88 members

by Rahul@gurome » Fri Nov 19, 2010 1:32 pm
Viper83 wrote:My concern with this question is the following: why is the square root of 9 equal to ONLY 3. The square root of 9 should be 3 and -3. Furthermore, -3 is not a prime number.

What am I missing here?
When we are dealing with square root(s), it is always the positive one.
By definition √(x²) = |x|.

This means, if x² = 9, and we are asked to find the value of x, the answer is -3 and 3. But if are asked to find the value of √(x²), the answer is only 3.
Rahul Lakhani
Quant Expert
Gurome, Inc.
https://www.GuroMe.com
On MBA sabbatical (at ISB) for 2011-12 - will stay active as time permits
1-800-566-4043 (USA)
+91-99201 32411 (India)
Top
Senior | Next Rank: 100 Posts
Posts: 54
Joined: Tue Feb 02, 2010 5:17 am
Thanked: 2 times

by gdk800 » Fri Nov 19, 2010 3:11 pm
Viper83 wrote:

Is Root of x a prime number?

(i) Abs (3x-7)= 2x+2

(ii) x to the power 2= 9x
Statement 1: |3x - 7| = (2x + 2)
Absolute value gives rise to two situations,

(1) For 3x > 7, (3x - 7) = (2x + 2) => x = 9 => √x = 3 -> Prime
or
(2) For 3x < 7, (7 - 3x) = (2x + 2) => x = 1 => √x = 1 -> Not Prime



Not sufficient.

Statement 2: x² = 9x
=> x² - 9x = 0 => x(x - 9) = 0 => x = 0 or 9

Not sufficient.

1 & 2 Together: x = 9 => √x = 3 -> Prime

Sufficient.



I have a question for Rahul, can't we ignore x = 0 in the second case because root(0) is not considered? By this, the ans. would be B.

Actually this brings me to another question as to how is root(0) treated?
Top

GMAT/MBA Expert

User avatar
GMAT Instructor
Posts: 1179
Joined: Sun Apr 11, 2010 9:07 pm
Location: Milpitas, CA
Thanked: 447 times
Followed by:88 members

by Rahul@gurome » Fri Nov 19, 2010 3:22 pm
gdk800 wrote:I have a question for Rahul, can't we ignore x = 0 in the second case because root(0) is not considered? By this, the ans. would be B.

Actually this brings me to another question as to how is root(0) treated?
Our target is to identify the correct option, not to force an option to be the correct one. If there is a logic to ignore x = 0, we can certainly do that. But as there is none, we can't ignore it. And root of zero is zero only, which is not a prime number.
Rahul Lakhani
Quant Expert
Gurome, Inc.
https://www.GuroMe.com
On MBA sabbatical (at ISB) for 2011-12 - will stay active as time permits
1-800-566-4043 (USA)
+91-99201 32411 (India)
Top
Post new topic Post Reply
• Page 1 of 1