is xyz prime?

Expert replies

blaster: Master | Next Rank: 500 Posts; Posts: 233; Joined: Mon Jun 09, 2008 1:30 am; Thanked: 5 times

by blaster » Fri Aug 13, 2010 1:27 am

Can someone give detailed explanation for this question? I can't get logic of this question

Is a product of three integers XYZ a prime?
(1) X=-Y
(2) Z=1

Like this post

Source: Beat The GMAT — Data Sufficiency | Fri Aug 13, 2010 1:27 am

kmittal82: Master | Next Rank: 500 Posts; Posts: 324; Joined: Mon Jul 05, 2010 6:44 amFollowed by: 3 members; Location: London; Thanked: 70 times

by kmittal82 » Fri Aug 13, 2010 1:57 am

(1) X = -Y

=> XYZ = - Y x Y x Z

this gives us a negative number, and negative numbers cannot be prime. Hence, sufficient to answer the question

(2) Z = 1

XYZ = XY x 1

A prime number is one with 2 factors, one of them being 1 and the other being the number itself

In this case, it has 3 factors, 1, X and Y.
If X = Y = 1, then the answer is 1 which is not a prime number, and for any other values of X and Y, the number has more than 2 factors. so it cannot be prime. Hence, sufficient to answer the question

Answer should be (D), can you give the OA please?

Like this post

clock60: Legendary Member; Posts: 759; Joined: Mon Apr 26, 2010 10:15 amFollowed by: 3 members; Thanked: 85 times

by clock60 » Fri Aug 13, 2010 3:01 am

to me the answer is C, but not sure
(1) x=-y,
(-y)(y)*z=-y^2*z, y=1, z=2, -1*2=-2 not prime
y=1, z=-2, -1*-2=2 prime
so 1 st not sufficient
(2)z=1, not sufficient as x,y not mentioned
both
-y^2*1 it can`t be prime as possible factors 1,-1 y
so my pick for C

Like this post

kmittal82: Master | Next Rank: 500 Posts; Posts: 324; Joined: Mon Jul 05, 2010 6:44 amFollowed by: 3 members; Location: London; Thanked: 70 times

by kmittal82 » Fri Aug 13, 2010 3:10 am

clock60 wrote:to me the answer is C, but not sure
(1) x=-y,
(-y)(y)*z=-y^2*z, y=1, z=2, -1*2=-2 not prime
y=1, z=-2, -1*-2=2 prime
so 1 st not sufficient
(2)z=1, not sufficient as x,y not mentioned
both
-y^2*1 it can`t be prime as possible factors 1,-1 y
so my pick for C

Ah, you are right clock60.

In my reasoning for (1), I missed out on the fact that Z could also be negative...

So yes, (1) is not sufficient.

However, with (2), even if X and Y are not mentioned, we still know that the number has 3 factors (1, X and Y), so it cant be prime... right?

Like this post

clock60: Legendary Member; Posts: 759; Joined: Mon Apr 26, 2010 10:15 amFollowed by: 3 members; Thanked: 85 times

by clock60 » Fri Aug 13, 2010 3:17 am

not completely agree z=1, (given) x=1, y=2 1*1*2=2 prime
z=1, x=2, y=2 1*2*2 =not prime

Like this post

kmittal82: Master | Next Rank: 500 Posts; Posts: 324; Joined: Mon Jul 05, 2010 6:44 amFollowed by: 3 members; Location: London; Thanked: 70 times

by kmittal82 » Fri Aug 13, 2010 4:00 am

clock60 wrote:not completely agree z=1, (given) x=1, y=2 1*1*2=2 prime
z=1, x=2, y=2 1*2*2 =not prime

Great, thanks for pointing that out (god I hate DS!

)

Like this post

clock60: Legendary Member; Posts: 759; Joined: Mon Apr 26, 2010 10:15 amFollowed by: 3 members; Thanked: 85 times

by clock60 » Fri Aug 13, 2010 4:01 am

kmittal82 wrote:
clock60 wrote:not completely agree z=1, (given) x=1, y=2 1*1*2=2 prime
z=1, x=2, y=2 1*2*2 =not prime
Great, thanks for pointing that out (god I hate DS! )

i hate it even greater!

Like this post

blaster: Master | Next Rank: 500 Posts; Posts: 233; Joined: Mon Jun 09, 2008 1:30 am; Thanked: 5 times

by blaster » Fri Aug 13, 2010 4:26 am

right answer is C.

Thanks guys!

Like this post

Post new topic Post Reply

• Page 1 of 1

GMAT Course Reviews

560 reviews

1168 reviews

431 reviews

359 reviews

147 reviews

5/5

5 Star (560 Reviews)

"Target Test Prep is the closest to the official version of the GMAT exam, about 99% accuracy in terms of the quality and quantity of information. The course has excellently created singular sets of focused lessons and tests for every possible topic that one could come across in the official GMAT exam."

"The TTP course maximizes the efficiency of the time you spend studying. It will take time and effort but I could almost guarantee that if you complete the course exactly as it is laid out you will get an amazing score. They also have a very responsive team willing to help with any questions you might have."

"TTP has two things that I think no other test prep company offers: A teaching approach that reinforces understanding and an attitude that will give you the mental preparedness needed to succeed on the test. TTP gives you a deep understanding of the concept you need to know while teaching you how to think."

Write a Review