on the numberline shown, is zero halfway btween r and s?

[suri_c]

data sufficiency problem.

sorry in advance! i am not a tech savvy person and do not know how to reproduce the diagram from the GMATPrep. I am hopeful that you can figure out from my description :-)

we are given a simple diagram of a numberline with 3 tickmarks: the first (the leftmost) tickmark labeled as r, the second one as s, the third one as t.
then the question reads, "On the numberline shown, is zero halfway between r and s?"
statement 1) s is to the right of zero.
statement 2) the distance between t and r is the same as the distance between t and -s.

ans: c

i thought statement 2 alone was sufficient. i don't know why it isn't :-( please help!

[Guest]

I also thought it was B, did you ever figure this one out? We have to assume that s > r because it it is to the right on the number line yes?

[Guest]

The basic idea: You cannot think of S as positive and -S as negative. If S is positive then -S is negative; but if S is negative then -S is positive :)
(1) identifies that S is +ve

[RonPurewal]

http://www.manhattangmat.com/forums/dis ... t4304.html

[sphinxodus]

Always remember that RST could be placed as follows
Case 1) Totally on the LHS of ZERO
---R---S----T-----0------
Case 2) Totally on the RHS of ZERO
---0----R---S----T-----
Case 3) Scattered across on the ZERO
---R----0---S----T-- or --R---S---0----T

(1) Only tells the position of ZERO and not the distances between. NOT SUFFICIENT

(2) "The distance between T and R is the same as the distance between T and -S"
Case 1) Not possible, as both R & S will be on one side of zero i.e. LHS
Case 2) Not possible, as both R & S will be on one side of zero i.e. RHS
Case 3)
a) ---R----0---S----T--
Means T+R = T+2S S = -R (YES)
b.1) --R---S---0------T |S| < |T|
Means T+R = T-S R = -S (YES)
b.2) --R---S-------0--T |S| > |T|
Means T+R = S-T 2T = S-R (NO)

Therefore, above cases leave us stranded, as one is a NO.

Combining (1) & (2), we are left with only Case 3, which says YES

[RonPurewal]

please post all future posts on this problem on the thread that i linked in the preceding post; we don't want to have multiple running threads on the same problem.
thanks.