DS Sequence.

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rakeshd347: Master | Next Rank: 500 Posts; Posts: 391; Joined: Sat Mar 02, 2013 5:13 amFollowed by: 4 members; Thanked: 50 times

by rakeshd347 » Fri Oct 04, 2013 11:42 pm

K is a set of numbers such that
(i) If x is in K, then -x is in K, and
(ii) if each of x and y is in K, then xy is in K
Is 12 in K?

(1) 2 is in K
(2) 3 is in K

OA is C

Last edited by rakeshd347 on Sat Oct 05, 2013 3:30 am, edited 1 time in total.

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Source: Beat The GMAT — Data Sufficiency | Fri Oct 04, 2013 11:42 pm

theCodeToGMAT: Legendary Member; Posts: 1556; Joined: Tue Aug 14, 2012 11:18 pmFollowed by: 34 members; Thanked: 448 times; GMAT Score:650

by theCodeToGMAT » Sat Oct 05, 2013 12:15 am

Set K ={ a, b, c, -a, -b, -c, ab, bc, ac, aab, abc, aac, abbc, abac, ........}
--> no relation between numbers..

To find --> 12 number in "K"

Statement 1:
a = 2
No, info of others..
INSUFFICIENT

Statement 2:
b = 3
No, info of others..
INSUFFiCIENT

Combining,,.
a = 2
b = 3
ab = 6
aab = 12

SUFFICIENT

Answer [spoiler]{C}[/spoiler]
What is the OA????

R A H U L

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vinay1983: Legendary Member; Posts: 643; Joined: Wed Aug 14, 2013 4:27 amFollowed by: 7 members; Thanked: 48 times

by vinay1983 » Sat Oct 05, 2013 1:10 am

rakeshd347 wrote:K is a set of numbers such that
(i) If x is in K, then -x is in K, and
(ii) if each of x and y is in K, then xy is in K
Is 12 in K?

(1) 2 is in K
(2) 3 is in K

If "a" is present "-a" is also present
If "a" is present and "b" is present then "ab" is present

Statement 1

"2" and "-2" are present, also we don't know "b" here. So insufficient

Statement 2

"3" and "-3" are present, also no info about "b". Insufficient

Combining 1 and 2

We know that 2, -2, 3, -3 are present in the set. But I think even this is not sufficient.

You can, for example never foretell what any one man will do, but you can say with precision what an average number will be up to!

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theCodeToGMAT: Legendary Member; Posts: 1556; Joined: Tue Aug 14, 2012 11:18 pmFollowed by: 34 members; Thanked: 448 times; GMAT Score:650

by theCodeToGMAT » Sat Oct 05, 2013 1:21 am

vinay1983 wrote: We know that 2, -2, 3, -3 are present in the set. But I think even this is not sufficient.

Vinay, it should be sufficient

In this question, i have a doubt.. whether the question means
==>SERIES 1: a , b , -a, -b, ab, aab, bab, .....
Or,
==>SERIES 2: a , b, -a, -b, ab

But, for both the cases.. taking your case 2, -2, 3, -3

SERIES 1: aab = (2)(2)(3) = 12 YES
SERIES 2: ab = (2)(3) = 6 NO.

So, whether you select any of the series.. you will have DEFINITE Answer.. I think Question Stem means SERIES 2 only.

So, Answer {C}

R A H U L

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rakeshd347: Master | Next Rank: 500 Posts; Posts: 391; Joined: Sat Mar 02, 2013 5:13 amFollowed by: 4 members; Thanked: 50 times

by rakeshd347 » Sat Oct 05, 2013 3:29 am

theCodeToGMAT wrote:
vinay1983 wrote: We know that 2, -2, 3, -3 are present in the set. But I think even this is not sufficient.
Vinay, it should be sufficient

In this question, i have a doubt.. whether the question means
==>SERIES 1: a , b , -a, -b, ab, aab, bab, .....
Or,
==>SERIES 2: a , b, -a, -b, ab

But, for both the cases.. taking your case 2, -2, 3, -3

SERIES 1: aab = (2)(2)(3) = 12 YES
SERIES 2: ab = (2)(3) = 6 NO.

So, whether you select any of the series.. you will have DEFINITE Answer.. I think Question Stem means SERIES 2 only.

So, Answer {C}

Answer is Indeed C. Rahul's reasoning is spot on.

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GMATGuruNY: GMAT Instructor; Posts: 15539; Joined: Tue May 25, 2010 12:04 pmFollowed by: 1906 members; Location: New York, NY; Thanked: 13060 times; GMAT Score:790

by GMATGuruNY » Sat Oct 05, 2013 3:46 am

K is a set of numbers such that
a) if x is in K, then -x is in K and
b) if each of x and y is in K, then xy is in K.

Is 12 in K?

1)2 is in K.
2)3 is in K.

If x is in K, then -x is in K.
In other words:
If a particular value is in K, then -(THAT VALUE) also is in K.
If x and y are in K, then xy is in K.
In other words:
If any two particular values are in K, then THEIR PRODUCT also is in K.

The conditions above apply to EVERY value in K.
Thus, each condition will yield an INFINITE number of values in K, as we will see when we evaluate the two statements.

Statement 1: 2 is in K
Thus, -2 is in K.
Thus, 2 * -2 = -4 is in K.
Thus, -(-4) = 4 is in K.
Thus, 2*4 = 8 is in K.
Thus, -2*4 = -8 is in K.
Thus, 4 * - 4 = -16 is in K.
Thus, -(-16) = 16 is in K.
Thus, K = {...-16, -8, -4, -2, 2, 4, 8, 16...}.
But we don't know what other values might be in K, so 12 might be in K or 12 might not be in K.
Insufficient.

Statement 2: 3 is in K
Thus, -3 is in K.
Thus, 3 * -3 = -9 is in K.
Thus, -(-9) = 9 is in K.
Thus, 3*9 = 27 is in K.
Thus, -3*9 = -27 is in K.
Thus, K = {...-27, -9, -3, 3, 9, 27...}.
But we don't know what other values might be in K, so 12 might be in K or 12 might not be in K.
Insufficient.

Statements 1 and 2 combined:
Since both 4 and 3 are in K, 4*3 = 12 is in K.
Sufficient.

The correct answer is C.

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