Logical ReasoningDifficulty: Hard

PT23 S2 Q10 Explanation

All bridges built from 1950 to

A free, expert breakdown of this official LSAT Logical Reasoning question.

TopicsMust be True

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Stimulus

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 the bridges that were built according to faulty engineering design.

What this question is testing

Must be True

Your task

Break the argument into its conclusion and evidence, then do exactly what the question stem asks with that structure.

Common trap

Answers that sound relevant to the topic but don't connect to the argument's actual reasoning.

Winning move

Predict what a right answer must do, then test each choice against the conclusion-evidence gap.

Reading along? Open the full official question in LawHub — we show a fragment here and keep the reasoning in our own words.

The question
10.

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

Answer choices

  1. Unsupported Distinction13% picked this

    Some suspension bridges are not in serious need

    Is it possible that this is false? Could it be that "all suspension bridges are in serious need of rehab"? Sure. The only thing we explicitly know about suspension bridges is that none of them were built according to faulty design (through implication we also know that none of them were built from 1950 to 1960). It's possible that suspension bridges weren't built according to faulty design but are still all in serious need of some rehab. So this answer doesn't have to be true. It could be false.

  2. Unprovable15% picked this

    Some suspension bridges are in serious need

    This is the flipside of our analysis of (A). Is it possible that "NO suspension bridges are in serious need of rehab"? Sure. All we know about suspension bridges is that they were all built according to good design. We have no idea what proportion of them are / aren't in need of serious rehab (something well designed will still need serious rehab at some point, depending on how old it gets). It's possible that none of them need serious rehab yet, that all of them are due for serious rehab, or any fraction in between.

  3. Correct63% picked this

    Some bridges that were built according to faulty engineering design are in serious

    Why this is right

    Do we know of at least one bridge that is both faulty design and in serious need of rehab? Yes, simply by combining the first two claims. If we know All A's are B and Some A's are C then we know Some things are B and C If we know All bridges from 1950-1960 badly need rehab and we know Some bridges from 1950-1960 had faulty design then we know Some of these bridges are both (faulty design) and (badly need rehab)

    Skill tested: Must be True · how this choice captures the argument's function is the move to repeat next time.

  4. Contradicted5% picked this

    Some bridges built from 1950 to 1960 are not in serious

    The first sentence says the exact opposite of this.

  5. Contradicted4% picked this

    Some bridges that were built according to faulty engineering design are not bridges other

    If we're saying "you're not a bridge other than a suspension bridge", we're saying "you ARE a suspension bridge". So this answer is saying Some bridges with faulty design are suspension bridges. This contradicts our final claim, that all suspension bridges do not have faulty design.

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