LSAT Must Be True Questions With Answers Practice Questions with Answers

Logical reasoning sections on the LSAT feature Must Be True questions as a foundational element of the exam's deductive logic requirements. These questions ask you to identify a statement that is a guaranteed logical consequence of the information provided in the stimulus. Unlike other question types that might ask for what is likely or what could be true, these demand 100% certainty based strictly on the text. Understanding how to tackle LSAT Prep materials involves recognizing that you cannot bring in outside information; the answer must be provable using only the premises given.

Concept Explanation

An LSAT Must Be True question is a logical reasoning task that requires you to select the answer choice that is a necessary conclusion derived from the provided premises. These questions test your ability to perform valid deductions and understand formal logic structures, such as conditional statements and quantifiers. In these problems, the stimulus is always treated as absolute fact, even if it describes a scenario that seems counterintuitive. The correct answer is often a paraphrased version of one of the premises or a combination of two or more premises using the transitive property or the contrapositive. Common indicators for these questions include stems like "Which one of the following must be true?", "Which one of the following can be properly inferred?", or "The statements above, if true, most strongly support which one of the following?" (though the latter sometimes allows for slightly less than 100% certainty, the approach remains similar). To succeed, you must avoid "could be true" traps, which are statements that are possible but not guaranteed by the text.

Solved Examples

1. Conditional Chain Example

   Stimulus: If a student studies for the LSAT, they will improve their score. Sarah studied for the LSAT.

   Step-by-Step Solution:

   1. Identify the conditional statement: $S \rightarrow I$ (If study, then improve).
   2. Identify the factual premise: $S$ (Sarah studied).
   3. Apply Modus Ponens: Since the sufficient condition $S$ is met, the necessary condition $I$ must occur.
   4. Conclusion: Sarah will improve her score.
2. Contrapositive Example

   Stimulus: Every professional athlete in the league follows a strict diet. Mark does not follow a strict diet.

   Step-by-Step Solution:

   1. Diagram the rule: $PA \rightarrow SD$ (If Pro Athlete, then Strict Diet).
   2. Form the contrapositive: $eg SD \rightarrow eg PA$ (If not Strict Diet, then not Pro Athlete).
   3. Apply the fact: Mark is $eg SD$.
   4. Conclusion: Mark is not a professional athlete in the league.
3. Quantifier (Some/Most) Example

   Stimulus: Most residents in the building own a car. Everyone who owns a car pays a monthly parking fee.

   Step-by-Step Solution:

   1. Identify the groups: "Most Residents" (more than 50%) are in the "Car Owner" group.
   2. Identify the second rule: All "Car Owners" are in the "Parking Fee" group.
   3. Combine the groups: Since most residents are car owners, and all car owners pay the fee, it follows that those specific residents must pay the fee.
   4. Conclusion: Most residents in the building pay a monthly parking fee.

Practice Questions

1. If the city builds a new stadium, taxes will increase. If taxes increase, the local economy will slow down. The city is currently building a new stadium. Which one of the following must be true?

2. All successful novelists are disciplined. Some successful novelists use digital typewriters. Which one of the following must be true?

3. No one who is allergic to nuts can eat the house salad. Jennifer is eating the house salad. Which one of the following must be true?

4. All mammals have lungs. All dolphins are mammals. Some dolphins live in the Atlantic Ocean. Which one of the following must be true?

5. If the alarm rings, the security guard will check the door. The security guard did not check the door. Which one of the following must be true?

6. Most students who attend the university live off-campus. All students who live off-campus must pay for their own utilities. Which one of the following must be true?

7. Only members of the board can vote on the new policy. Sarah is not a member of the board. Which one of the following must be true?

8. Any company that adopts the new software will see an increase in productivity. Company X did not see an increase in productivity. Which one of the following must be true?

Answers & Explanations

1. The local economy will slow down. This follows from a transitive chain: $Stadium \rightarrow Taxes \rightarrow Slowdown$. Since the stadium is being built, the end of the chain is guaranteed.
2. Some people who use digital typewriters are disciplined. Since some novelists use digital typewriters and all novelists are disciplined, the overlap between these two groups is a logical necessity.
3. Jennifer is not allergic to nuts. The rule states $Allergic \rightarrow eg Salad$. The contrapositive is $Salad \rightarrow eg Allergic$. Since Jennifer is eating the salad, she cannot be allergic.
4. Some animals with lungs live in the Atlantic Ocean. Since all dolphins are mammals and all mammals have lungs, all dolphins have lungs. Because some dolphins live in the Atlantic, those specific dolphins are animals with lungs.
5. The alarm did not ring. The original rule is $Alarm \rightarrow Check$. The contrapositive is $eg Check \rightarrow eg Alarm$. Since the guard did not check, the alarm could not have rung.
6. Most students who attend the university pay for their own utilities. This combines the "most" group with an "all" group. If more than 50% of students live off-campus, and 100% of those people pay utilities, then at least that same "more than 50%" must pay utilities.
7. Sarah cannot vote on the new policy. The term "only members" establishes that being a board member is a necessary condition for voting ($Vote \rightarrow Member$). Without the necessary condition, the sufficient condition (voting) cannot occur.
8. Company X did not adopt the new software. This is the contrapositive of the rule $Adopt \rightarrow Productivity$. If productivity did not increase, the software adoption could not have happened.

Frequently Asked Questions

What is the difference between "Must Be True" and "Most Strongly Supported"?

Must Be True questions require a conclusion that is 100% logically guaranteed by the premises, whereas Most Strongly Supported questions allow for a conclusion that is highly probable but not strictly certain. While the strategies are similar, the threshold for the correct answer is slightly lower for the latter.

Can the correct answer be a simple repeat of a premise?

Yes, on the LSAT, a premise itself is technically something that must be true based on the stimulus. While rare, sometimes the correct answer is just a slightly paraphrased version of a fact already stated in the text.

How do I handle "except" questions in this category?

In a "Must Be True EXCEPT" question, four of the answers will be logically guaranteed by the stimulus. The correct answer will be the one that either "Could Be True" or "Must Be False," as long as it is not a necessary consequence of the premises.

Should I use outside knowledge to solve these?

No, you must never use outside knowledge for LSAT Must Be True questions. Even if a premise states something scientifically impossible, like "all dogs are reptiles," you must accept it as fact and choose an answer that follows from that specific logic.

What are common trap answers?

Common traps include "Could Be True" answers, which are possible but not certain, and "Exaggerated" answers, which take a premise about "some" people and apply it to "all" people. Using the AI Question Generator can help you practice identifying these subtle differences in wording.