Must Be True Questions in Logical Reasoning

Must Be True Questions in Logical Reasoning

  • About 4% of all questions in logical reasoning are must be true questions.
  • They set a good foundation for how to think about logical reasoning.
  • The complexity involved with these questions typically tends to make them harder than normal.

Identifying Must Be True Questions

  • Key phrases in question stems:
    • "If the statements above are true, which one of the following must also be true?"
    • "Which one of the following can be logically inferred from the statements above?" (Inferred = must be true)
    • "Which one of the following can be properly concluded from information given?" (Concluded = inferred = must be true)
    • "Find a consequence of the view above" (Consequence = follows from the view = must be true)
    • "If the statements in the passage are true, each of the following must be true except" (Find the one that doesn't have to be true).

Process for Answering Must Be True Questions

  1. Find the related statements:
    • Statements will have something in common, linking them together.
    • Look for terms repeated across sentences.
  2. Understand the type of reasoning:
    • Is it an if/then relationship (conditional logic)?
    • Is it a comparative relationship (bigger than, faster than)?
  3. Anticipate/Prephrase:
    • What could the statements imply?
  4. Work through answer choices:
    • Generally work from wrong to right.
    • Use trap answer patterns to help eliminate.

Conditional Logic Example

  • Conditional logic is a major player in must be true questions. 75% of must be true questions contain if/then conditional logic.
  • If you have two if/then statements with a common term, you can connect them using the transitive property.
  • Example:
    • If A, then B
    • If B, then C
    • Inference: If A, then C
  • This is a hypothetical syllogism, one of the most common structures on the LSAT.
Example
  • Statements:
    • If K, then M
    • If P, then not M
    • L, then K
  • Question: Which of the following must be true?
    • A) If K, then P (Incorrect)
    • B) If P, then not L (Correct)
      • If P, then not M.
      • If not M, then not L and not K. Therefore, If P, then not L.
    • C) If M, then L (Incorrect)
Real LSAT Question Example
  • Statements:
    • If the price it pays for coffee beans continues to increase, the coffee shop will have to increase its prices.
    • In that case, either the coffee shop will begin selling non coffee products or its coffee sales will decrease.
    • But selling non coffee products will decrease the coffee shop's overall profitability.
    • Moreover, the coffee shop can avoid a decrease in overall profitability only if its coffee sales do not decrease. (If sales decrease, then decreases profits.)
  • Inference: If the price paid for beans goes up, then they're gonna have a decrease in profits no matter what.
  • Answer choices:
    • A) If coffee shop's overall profitability decreases, the price it pays for coffee beans will have continued to increase. (Reversal - incorrect)
    • B) If the coffee shop's overall profitability decreases, either we'll have begun selling non coffee products, whereas coffee sales will have decreased. (Another reversal - incorrect)
    • C) The coffee shop's overall probability will decrease if the price it pays for coffee beans continues to increase. (Correct)
    • D) The price it pays for coffee beans cannot decrease without the coffee shop's overall profitability also decreasing. (Unsupported relationship - incorrect)
    • E) Either the price pays for coffee beans will continue to increase or the coffee shop's coffee sales will increase. (Negation - incorrect)

Comparison Example

  • 19% of must be true questions involve comparative reasoning.
  • Example:
    • City A is more populous than City B.
    • City A is geographically smaller than City B.
    • Inference: The density of City A is much greater than the density of City B.
  • Comparisons typically take one of two forms on the LSAT:
    • Comparing two different things at a point in time
    • Comparing one thing over two different points in time
Example
  • Statement: No headache pill stops pain more quickly than Dinaxol.
  • Explanation: Dinaxol is the fastest or tied for the fastest. The statement doesn't tell us how fast Dinaxol is, only that no competitor is faster.
  • Answer choices:
    • A) Evelyn's headache pain will be relieved, but Jane's will not. (Contradicted - incorrect)
    • B) Evelyn's headache pain will be relieved more quickly than James. (Very tempting but incorrect)
    • C) Evelyn's headache will be relieved at least as quickly as James. (Correct)
    • D) Chain static pain will be relieved at the same time as Evelyn's. (Unsupported - incorrect)
    • E) Jane will be taking Dinexil for relief from headache pain. (Unsupported - incorrect)

Causation Example

  • Causation is rare on must be true questions, but it can occur.
Example
  • Statements:
    • The calm shallow waters of coastal estuaries are easily polluted by nutrient rich sewage. (Sewage causes pollution)
    • When estuary waters become overnutrified, as a result, algae pro proliferate. (Pollution causes algae)
    • The abundant algae in turn sometimes provide a rich food source for microorganisms that are toxic to fish. (Algae cause toxic organisms)
    • Thereby killing most of the fish in the estuary. (Toxic organisms kill fish)
  • This presents a causal chain. Causal chains allow us to make inferences and moving along the causal chain in a forward direction.
  • Answer choices:
    • A) Fish in an estuary that has been polluted by sewage are generally more likely to die from pollution than are fish in an estuary that has been polluted in some other way. (Unsupported - incorrect).
    • B) In estuary waters that contain abundant algae, microorganisms that are toxic to fish reproduce more quickly than other types of microorganisms. (Unsupported - incorrect).
    • C) Nutrients and other components of sewage do not harm fish and coastal estuaries in any way other than through the resulting proliferation of toxic microorganisms? (Out of scope - incorrect).
    • D) Algae will not proliferate in coastal estuaries that are not polluted by nutrient rich sewage. (Negation - incorrect).
    • E) Overnutrifying estuary waters by sewage can result in the death of most of the fish in the estuary. (Correct).

Trap Answer Patterns

  • Scope:
    • Out of scope.
    • Mere speculation.
  • Logic:
    • Unsupported relationships (wrong type of reasoning or randomly connecting terms).
    • Opposite of what you're looking for (something that must be false).
    • Contradict the statements.
    • Reverse or negate conditionals or causal chains.
    • Lend support to one of the statements (but don't follow from them).
  • Degree:
    • Too strong.

How to Spot a Must Be True Question

  • Look for language such as "must be true," "inferred," "follows logically," or "can be concluded."

Reasoning Structures

  • Conditional logic is the most common.
  • Comparison and causation also play roles.

Trap Answer Patterns to Watch Out For

  • Building relationships in the wrong type of reasoning or connecting the wrong terms.
  • Something stronger than what you know.
  • Reverse or negate.
  • Contradict the statements.