All bridges built from 1950 to 1960 are in serious need of rehabilitat

Question

All bridges built from 1950 to 1960 are in serious need of rehabilitation. Some bridges constructed in this period, however, were built according to faulty engineering design. That is the bad news. The good news is that at least some bridges in serious need of rehabilitation are not suspension bridges, since no suspension bridges are among the bridges that were built according to faulty engineering design.

If the statements above are true, then, on the basis of those statements, which one of the following must be true?

(A) Some suspension bridges are not in serious need of rehabilitation.

(B) Some suspension bridges are in serious need of rehabilitation.

(C) Some bridges that were built according to faulty engineering design are in serious need of rehabilitation.

(D) Some bridges built from 1950 to 1960 are not in serious need of rehabilitation.

(E) Some bridges that were built according to faulty engineering design are not bridges other than suspension bridges.

LGOdream · Accepted Answer

I think the key to this problem lies in the " at least  some  bridges in serious need of rehabilitation are not suspensions bridges ". Could be some, could be all. If all  , only C stands as true.

Xmarksthespot · Answer

The question asks for what MUST be true based on the information given. For formal logic questions like this one, it is very helpful, and maybe even necessary, to diagram the relationships between and among the different elements. The diagram for this stimulus is as follows:

Faulty <--some--> built 1950-60 --all--> in serious need of rehab --some are not --> suspension bridges <--none--> faulty

Now let's see which of the answer choices MUST BE TRUE according to our diagram.

A) Some suspension bridges are not in serious need of rehabilitation.

- Nope, not necessarily. We know that some bridges in serious need of rehab are not suspension bridges, but this does not necessarily mean that some suspension bridges are not in serious need of rehab. To simplify further, suppose there are 10 bridges in serious need of rehab. 3 of them are not suspension bridges (satisfies the requirement that some are not suspension bridges). What about the other 7? Could 1 or more or even all of them be suspension bridges? Absolutely. Now what if there are only 7 suspension bridges, and they all happen to be the 7 that need rehab? Then we can say that all suspension bridges are in serious need of rehab. Hence, A does not necessarily have to be true.

(B) Some suspension bridges are in serious need of rehabilitation.

- Again, this does not necessarily have to be true. In fact, there is nothing in the stimulus that supports this. This could be true, it could be false.

(C) Some bridges that were built according to faulty engineering design are in serious need of rehabilitation.

- This is the correct answer. Looking at our diagram, we see that some bridges that were built according to faulty design where built between 1950-1960, and all bridges built between 1950 and 1960 are in serious need of rehab. Hence, it follows that some bridges built according to faulty design are in serious need of rehab.

One can argue that the stimulus only states that some bridges built between 1950-1960 are in serious need of rehab, and not the other way around. But consider the following scenario: 10 bridges were built during 1950-1960. 3 of them were built according to faulty design (satisfying the requirement in the stimulus). From this, we can clearly see that at least 3 bridges built according to faulty design were built during 1950-1960. Hence, some bridges built according to faulty design were built during 1950-60.

(D) Some bridges built from 1950 to 1960 are not in serious need of rehabilitation.

- This directly contradicts the information in the stimulus and is therefore false.

(E) Some bridges that were built according to faulty engineering design are not bridges other than suspension bridges.

- Nothing in the stimulus supports this. Could be true, could be false. We are looking for the answer that must be true.

u0422811 · Answer

B is not wrong as it is true that some bridges will need rehabilitation. It is just not as true as C which goes into greater detail saying that some bridges that were built according to faulty engineering design are in serious need of rehabilitation.

So C is just more precise than B.

So C is just more precise than B.

adishail · Answer

We have to assume that the choices refer to bridges between 1950 to 1960

Wrong, since all bridges from 1950 to 1960 need rehab  (A) Some suspension bridges are not in serious need of rehabilitation.

Very clearly given in the statement. Agreed it is TRUE, but then it has already been stated in the statement that "... some bridges in serious need of rehabilitation are not suspension bridges ..."  (B) Some suspension bridges are in serious need of rehabilitation.

(C) Some bridges that were built according to faulty engineering design are in serious need of rehabilitation.

Wrong, since specified that all bridges from 1950 to 1960 need rehab  (D) Some bridges built from 1950 to 1960 are not in serious need of rehabilitation.

Wrong, not bridges other than suspension bridges means - suspension bridges  (E) Some bridges that were built according to faulty engineering design are not bridges other than suspension bridges.

Going back to the remaining choices, C can be inferred and B is given fact (does not require inference).

TehJay · Answer

Another thing wrong with (B) is that the question is asking which one MUST be true. The article only claims that " at least  some bridges... are not suspension bridges." That means it's possible that NONE of the bridges are suspension bridges, in which case (B) would be false.

fluke · Answer

honey1 · Answer

hey can you plz help me with this question
from the 1st line of the stimulus it is given that all bridges need rehabilitation which means this 'all' include suspension bridges also, so if it include suspension bridges then how can option 2nd is wrong as if all bridges need rehab then it include the suspension bridge also

GMATGuruNY · Answer

Apply the NEGATION TEST.
When the correct answer is negated, the passage will be contradicted.
Note:
The opposite of SOME X is NO X.

C, negated:
 No bridges that were built according to faulty engineering design are in serious need of rehabilitation .
This negation contradicts the following information in the passage:
 All bridges built from 1950 to 1960 are in serious need of rehabilitation.
Some bridges constructed in this period were built according to faulty engineering design. 
Since the negation of C contradicts the passage, C is a valid INFERENCE: a statement that MUST BE TRUE, given the information in the passage.

C

B, negated:
 No suspension bridges are in serious need of rehabilitation. 
Since the passage states that  at least some bridges in serious need of rehabilitation are not suspension bridges , it is possible that ALL bridges in need of rehabilitation are not suspension.
In other words, it is possible that NO suspension bridges are in need of rehabilitation.
Since the negation of B does not contradict the passage, B is NOT a valid inference.
Eliminate B.

AndrewN · Answer

Hello,  honey1 . We seem to keep crossing paths on LSAT questions. I would like to say, first off, that I agree with the excellent analysis provided above by  GMATGuruNY . My take on answer choice (B) is as follows.

By way of analogy, we can think of a passage describing the inhabitants of Earth. The first half of line 4 from the passage could be adapted to say,  at least some inhabitants of Earth are not Martians . Does this mean that an appropriately altered answer (B) would be accurate?

Some Martians are inhabitants of Earth.

No, we cannot comment on a group about which we have no tangible information. We are only qualified to comment on this other group of  some inhabitants of Earth . Likewise, in the question at hand, all that we can say for certain is that  at least some bridges in serious need of rehabilitation are not suspension bridges , nothing more. Furthermore, we cannot put a finger on information from the passage to indicate that any of the suspension bridges mentioned were built from 1950 to 1960. This fact would make suspension bridges a red herring in the passage.

I hope that helps. Good luck with your studies.

- Andrew

honey1 · Answer

I think that I was making mistake by considering 1st line of stimulus as quite important but they are just providing me the background of the stimulus.
THANKS GMATGuruNY and AndrewN you explanation helped me a lot.

VerbalBot · Answer

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All bridges built from 1950 to 1960 are in serious need of rehabilitat

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All bridges built from 1950 to 1960 are in serious need of rehabilitat

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