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DS problem -

by cardoc » Sun Nov 06, 2011 5:20 pm
What is the value of v^3 - k^3?
(1) v k > 0
(2) v - k = 6

A. Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.
B. Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.
C. BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.
D. EACH statement ALONE is sufficient.
E. Statements (1) and (2) TOGETHER are NOT sufficient.

Answer - B

Can't figure out how the answer is B
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Source: — Data Sufficiency |
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by neelgandham » Sun Nov 06, 2011 5:32 pm
In my opinion, the answer is E. Can you let us know the source of the question please?
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by cardoc » Sun Nov 06, 2011 8:02 pm
Hi,

This is from 'GMAT SETS" from esnips.com
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by dabral » Sun Nov 06, 2011 9:23 pm
This question is in the Official GMATPrep software and the correct answer is E. The trap answer is B. The test writers are testing students who incorrectly assume that (v-k)^3 and v^3-k^3 are identical.

The correct answer E can be shown with the following examples.
Example#1: v=8, k=2, value of v^3-k^3= 8^3-2^3=512-8=504
Example#2: v=10, k=4, value of v^3-k^3 = 10^3 - 4^3=1000-64=936
Two distinct values, therefore E.
cardoc wrote:What is the value of v^3 - k^3?
(1) v k > 0
(2) v - k = 6

A. Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.
B. Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.
C. BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.
D. EACH statement ALONE is sufficient.
E. Statements (1) and (2) TOGETHER are NOT sufficient.

Answer - B

Can't figure out how the answer is B
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by Anurag@Gurome » Sun Nov 06, 2011 9:58 pm
cardoc wrote:What is the value of v^3 - k^3?
(1) v k > 0
(2) v - k = 6

A. Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.
B. Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.
C. BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.
D. EACH statement ALONE is sufficient.
E. Statements (1) and (2) TOGETHER are NOT sufficient.

Answer - B

Can't figure out how the answer is B
v^3 - k^3 = (v - k)(v² + vk + k²) = (v - k)(v² - 2vk + 3vk + k²)
= (v - k)(v² - 2vk + k² + 3vk)
= (v - k)((v - k)² + 3vk)

(1) vk > 0 just implies that vk is positive not the exact value of vk; NOT sufficient.

(2) v - k = 6 implies v^3 - k^3 = 6 * (6² + 3vk)
But we do not know the value of vk; NOT sufficient.

Combining (1) and (2) again we do not know the exact value of vk; NOT sufficient.

The correct answer is E.
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by cardoc » Mon Nov 07, 2011 10:05 am
Thanks everyone !!!
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by bpdulog » Mon Nov 07, 2011 11:49 am
I guessed E.
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