Fence X is twice as long as fence Y, and fence Y is 2 feet shorter tha

Question

Fence X is twice as long as fence Y, and fence Y is 2 feet shorter than fence Z. If 3 feet were added to each fence, which of the following must be true?

I) X is twice as long as Y.
II) Y is 2 feet shorter than Z.
III) X is longer than Z.

A) I only
B) II only
C) III only
D) I and II
E) II and III

Narenn · Answer

Length of Z = z
Length of Y = z-2
Length of X = 2z-4

Minimum possible length of fence Z is 2 feet, because if we take the value for length of z below 2, lengths of Z and Y would be negative which is not possible.

If Z = 2
then Y = 0 and X = 0

When we add 3 feet in each fence, we will get
Z= 5
Y=3
X=3

Only Statement II holds true with above values.

Maxirosario2012 · Answer

Fence X is twice as long as fence Y, and fence Y is 2 feet shorter than fence Z. If 3 feet were added to each fence, which of the following must be true?

I) X is twice as long as Y. 
 II) Y is 2 feet shorter than Z. 
 III) X is longer than Z.

A) I only 
 B) II only 
 C) III only 
 D) I and II 
 E) II and III

Thank you Narenn. 
I posted this problem because Kaplan suggest to use the "picking numbers" strategy.
But I think that backsolving is not the best approach.
Why?:

If X= 6
 Y= 3
 Z= 5

Then, after adding 3 feet to each:

X= 9
Y=6
Z=8

And the answer would be E, which is wrong.

GyanOne · Answer

X = 2Y and Z = Y+2 are given to us to be true according to the question.

1) (X+3) = 2(Y+3) => X + 3 = 2Y + 6 => X = 2Y +3, which cannot be true as X=2Y. NEVER TRUE--> REJECT. 
2) (Y+3) + 2 = Z+3 => Y+2 = Z, which is true. ALWAYS TRUE --> ACCEPT. 
3) X+3 > Z+3 => X>Z => 2Y > (Y+2) => Y>2. So this is true only if Y>2, else it is false. We can check this out quickly. If X=2, Y=1, Z=4, then X<Z, but if X=6,Y=3,Z=5, then X>Z. CONDITIONALLY TRUE --> REJECT.

Therefore the answer must be (B).

KarishmaB · Answer

You are correct. Number plugging in almost never a good approach when you have to prove that something MUST BE TRUE. To prove that something will be true in every case, how many cases are sufficient? Can I check 2 cases and say that it will be true in every case? Should I check 6, or 20? Even if something is true in 20 cases, it may not hold for the 21st case! We must use logic to prove that something must be true in every case.
But we can use number plugging to disprove something. i.e. we try to find the case in which it doesn't hold and then we can say that something needn't be true in every case.

Just taking the example of X = 6 (you took one case), you cannot say that II and III must be true. All you can say is that I certainly is not true in every case.

Using logic:
"X is twice as long as fence Y"
"fence Y is 2 feet shorter than fence Z"
"3 feet were added to each fence"

I) X is twice as long as Y.
X is originally twice as long as Y. When we add 3 to both, X will no longer be twice. Not true.

II) Y is 2 feet shorter than Z.
Originally, Y was 2 feet shorter than Z. If you add equal length to both (i.e. 3 feet) , Y will still remain 2 feet shorter than Z. Always true.

III) X is longer than Z.
X is twice of Y (hence X greater than Y) and Z is 2 more than Y (Z is greater than Y too). Both X and Z are greater than Y so we don't know what is the relation between X and Z.
Say if Y = 1, Z = 3 and X = 2. So X needn't be longer than Z.

Temurkhon · Answer

if Z=2, Y=0 and X=0 then X/Y not equal 2, so minimal cases is Z=3, Y=1 and X=2. The critical is that if X<Z or X=Z these facts make III wrong

glagad · Answer

Nowhere in the question have they mentioned Z's length. How are we even inferring "Z is 2 more than Y (Z is greater than Y too)". They only talk about Z's height, right?
Please let me know if I am missing something.

Fence X is twice as long as fence Y, and fence Y is 2 feet shorter tha

Prep Toolkit

Top 5 GMAT Debrief Videos

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards

GMAT Problem Solving (PS) Questions

Fence X is twice as long as fence Y, and fence Y is 2 feet shorter tha

Prep Toolkit

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards