manhattan challenge problem_DS_is n < 1

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nh8404052006: Senior | Next Rank: 100 Posts; Posts: 48; Joined: Wed Aug 27, 2008 9:58 am

by nh8404052006 » Sat May 02, 2009 6:32 am

Is n < 1 ?
(1) nx – n < 0
(2) x–1 = –2

OA: C
Please explain, thank

Source: Manhattan challenge problem

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Source: Beat The GMAT — Problem Solving | Sat May 02, 2009 6:32 am

cramya: Legendary Member; Posts: 2467; Joined: Thu Aug 28, 2008 6:14 pmFollowed by: 11 members; Thanked: 331 times

by cramya » Sat May 02, 2009 6:59 am

I am getting E

Stmt I

nx-n<0

n(x-1)<0

n->positive greater than 1 and x-1<0 then NO n is not less than 1

n-> negative and x-1>0 then YES n<1

INSUFF

Stmt II

x-1 = -2

No idea what n could be

TOGETHER:

n (x-1) < 0

x-1 = -2

-2n < 0

All we can say is n>0
n = 10000000 NO
n=1/2 YES

Must be missing a trick if its C

Regards,
CR

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dumb.doofus: Master | Next Rank: 500 Posts; Posts: 435; Joined: Sat Sep 27, 2008 2:02 pmFollowed by: 1 member; Location: San Jose, CA; Thanked: 43 times; GMAT Score:720

by dumb.doofus » Sat May 02, 2009 7:05 am

Just my opinion..

I think the above problem makes more sense if

a) = nx - x instead of nx - n

if it is nx - x then the answer is definitely C..

Can you confirm the above please?

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abhinav85: Master | Next Rank: 500 Posts; Posts: 179; Joined: Mon Dec 29, 2008 4:41 am; Thanked: 2 times

by abhinav85 » Sat May 02, 2009 7:21 am

IMO E for me too.

To find out if n<1 we have to find out the value of n?

From A we cannot say that n is positive or negative..becoz we
don't know the value of x.(insufficient)

From B we get the value of x that is -1.(insufficient)

Now when we take them together and put the value of
x in nx-n< 0
= -n-n < 0
=-2n<0

now if we take n as negative that is say -1 we get n> 0.

and if we take n as postive say 1 we get n < 0.

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nh8404052006: Senior | Next Rank: 100 Posts; Posts: 48; Joined: Wed Aug 27, 2008 9:58 am

by nh8404052006 » Sat May 02, 2009 7:26 am

From statement (2) ==>x = – 1 , but no info abt n ==> insufficient
From statement (1) ==> n (x – 1 ) < 0 . If n = – 1 and x =2 , n (x – 1 ) < 0 but when n = 2 and x = 0 , then n (x – 1 ) < 0 ==> insufficient
Both statements together ==>if x = – 1, and n (x – 1 ) < 0 ==> n > 0 ==>sufficient
Why? when the red portion is negative, the blue portion MUST be positive; otherwise, the inequaility( <0 )does not hold

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