Volume II: Logical Reasoning · Chapter 8
The job: Read It. Accept the stimulus as true and pick the answer the text proves.
Frequency: roughly 2 to 4 per LR section. Difficulty: low to high (the stems are easy to spot; the hard ones bury the inference in a combination).
A Must Be True question hands you a block of text and asks which answer choice the text proves. Your job is narrow and strict: treat every word in the stimulus as true, add nothing from your own knowledge, and find the one answer that follows. You are not evaluating an argument. You are not deciding whether anything in the stimulus is actually correct in the real world. You take the stimulus as a given and ask, “Granting all of this, what else do I now know?”
This is a Read It job. The answer is already lying in the text, either stated in different words or sitting one safe step away. You never need outside information and never have to guess at the author’s intent. Everything you need is on the page.
Two features set Must Be True apart from the argument-based types you will meet later. First, the stimulus is often a fact set, not an argument. It may be a pile of statements with no conclusion drawn. Do not waste time hunting for a conclusion or a gap that is not there. That hunt is a time sink, and it breeds tempting wrong answers because you start treating the “missing” piece as something to supply. Second, right answers are unpredictable. A rich fact set can support many valid inferences, and the writers can pick any of them. Unlike Weaken or Assumption, where a sharp prephrase often nails the answer, here you win mostly by elimination. Form a loose expectation, then test the five choices against the text.
The stem asks the statements to support an answer. Watch for:
Two look-alikes cause real damage on test day. Learn the boundaries cold.
One more calibration. “Must be true” is airtight; “most strongly supported” allows a tiny leap. When the stem says “must,” the answer must be guaranteed, with no daylight. When it says “most strongly supported,” the best answer may fall a hair short of certainty but is still clearly the most provable of the five. The reading is the same either way; only the strictness you demand of the survivor shifts. In both, the real job is identical: separate the one defensible answer from four indefensible ones.
1. Read the stem and confirm the job. Make sure the stimulus is supporting the answer, not the reverse. Note whether it says “must be true” (airtight) or “most strongly supported” (small leap allowed).
2. Read the stimulus closely, in its exact terms. Precision is the whole game here. “May happen” is not “will happen.” “Becoming less popular” is not “being unpopular.” “A large bill” is not “a large debt.” Do not smooth the text into a tidier paraphrase in your head, because the tidy version erases the exact wording the right answer depends on. Everyday version: a sign reading “up to 50% off” never promised that any one item is 50% off, so a shopper who hears “50% off” has quietly upgraded the claim, and that upgrade is the same swap that turns a true stimulus into a false answer.
3. Find statements that share a term and can combine. The richest inferences come from two statements that connect through a shared idea. Circle quantity words (all, most, some, none), conditional triggers (if, only, unless), and comparison words. Then look for two statements that touch the same term, and chain them. Use the tools from the foundation chapters:
A → B plus ¬B gives you ¬A (run the contrapositive). A → B plus B → C gives you A → C (the chain). These two patterns generate a large share of all credited Inference answers.most A are B plus most A are C gives you some B are C. A some statement reverses (some A are B means some B are A); most and all do not reverse into equally strong statements. Remember the existence caveat for all: it gives no reverse “some” inference unless you separately know at least one A exists.4. Prephrase loosely, then go to the answers. Note any clean inference you spotted, but hold it lightly. The test may credit a different valid inference, so do not reject a good answer just because it is not the one you predicted.
5. Eliminate with the point-to-it test. This is the signature move. If you cannot point to the exact text that proves an answer, kill it. For each surviving choice, put your finger on the sentence (or the two sentences) that guarantee it. If your finger lands on nothing, or on “well, that seems reasonable,” the answer fails. The credited answer always has a home in the text. The right answer takes one of two forms:
If an answer is neither a faithful paraphrase nor a valid combination, it cannot be proven, and it is wrong no matter how plausible it sounds.
6. Watch for the normative red flag. A purely factual stimulus cannot prove a value judgment. If every statement is a fact (what is, what happens, what was measured) and an answer says something should be done, or that something is good, bad, better, or wrong, be deeply suspicious. Facts do not yield “ought.” An answer that smuggles in a value word almost always fails the point-to-it test, because no sentence in a factual stimulus establishes a value.
A regional transit authority studied its bus routes over one year. Every route that was profitable in that year ran at least one express service during morning rush hour. No route that lacked dedicated bus lanes ran an express service at any time of day. Most of the authority’s routes that served the downtown core were profitable that year. The authority operates routes both inside and outside the downtown core.
If the statements above are true, which one of the following must also be true?
(A) Most of the authority’s routes that served the downtown core had dedicated bus lanes.
(B) Any route with dedicated bus lanes was profitable that year.
(C) Every profitable route served the downtown core.
(D) Some routes outside the downtown core were profitable that year.
(E) A route that ran an express service during morning rush hour was profitable that year.
Set up the conditionals. Translate the rules using Chapter 3 tools. “Every profitable route ran an express service” is Profitable → Express. “No route that lacked dedicated lanes ran an express service” is ¬Lanes → ¬Express, whose contrapositive is Express → Lanes. Chain them: Profitable → Express → Lanes, so Profitable → Lanes. Also note the quantifier fact: most downtown routes were profitable.
Combine and prephrase. Most downtown routes are profitable, and every profitable route has lanes (Profitable → Lanes). So most downtown routes have lanes. That is a clean combination inference. Hold it.
Test the answers with the point-to-it test.
Profitable → Lanes. If most downtown routes are profitable, and every profitable route has dedicated lanes, then most downtown routes have dedicated lanes. This is a valid combination of a quantifier fact and a conditional chain. It must be true.Profitable → Lanes, not Lanes → Profitable. Saying every route with lanes was profitable is the Mistaken Reversal of what you proved (see Chapter 3). Having lanes is necessary for being profitable, not sufficient for it. A route could have lanes and still lose money. Cannot point to it.Profitable → Express), not that morning-express routes were profitable (Express → Profitable). That is a Mistaken Reversal. A route could run a morning express and still be unprofitable. Cannot point to it.The survivor is (A), and you can put your finger on the exact two facts that prove it.
Decide which answer must be true (or is most strongly supported). Try each cold before reading the answers below.
1. A library tracked its collection. Every book in the rare-books room is kept behind glass. No book that circulates to patrons is kept behind glass. Some history books are in the rare-books room.
Which must be true?
(A) Some history books do not circulate to patrons.
(B) Every history book is kept behind glass.
(C) No history book circulates to patrons.
(D) Some books behind glass circulate to patrons.
(E) The rare-books room contains only history books.
2. A survey of one town found that most residents who own a bicycle commute by car. Most residents who own a bicycle subscribe to the local newspaper.
Which is most strongly supported?
(A) Most car commuters subscribe to the local newspaper.
(B) Some bicycle owners commute by car and subscribe to the local newspaper.
(C) Everyone who subscribes to the newspaper owns a bicycle.
(D) Some bicycle owners subscribe to the local newspaper.
(E) Most newspaper subscribers own a bicycle.
3. A new agricultural method increased yields on every farm that adopted it and had clay-rich soil. Farm Q adopted the method but saw no increase in yield.
Which must be true?
(A) Farm Q does not have clay-rich soil.
(B) The method never works on sandy soil.
(C) Farm Q should switch methods next season.
(D) Most farms that adopted the method had clay-rich soil.
(E) Clay-rich soil guarantees higher yields.
1. Answer: (A). Translate: rare-books → behind glass; circulates → not behind glass (contrapositive: behind glass → does not circulate). Chain: rare-books → behind glass → does not circulate. So any book in the rare-books room does not circulate. Some history books are in the rare-books room, so some history books do not circulate. Point to it: the “some history books in the rare-books room” fact plus the chain. (B) exaggerates “some” history books into “every” history book; history books outside the rare-books room are unconstrained. (C) overreaches to “no history book”; only the ones in the rare-books room are covered, and some history books may sit elsewhere and circulate. (D) is the opposite of a stated rule (behind glass → does not circulate). (E) is new information; nothing says the room holds only history books.
2. Answer: (B). Two “most” statements share the same base group: bicycle owners. Most bicycle owners commute by car, and most bicycle owners subscribe. Two majorities of the same group must overlap (Chapter 4), so at least one bicycle owner both commutes by car and subscribes to the local newspaper. That is exactly (B). (A) is too strong: even if a majority of bicycle owners commute and a majority subscribe, car commuters could include many people who do not own bicycles, so we cannot conclude that most car commuters subscribe. (C) reverses and exaggerates a “most” into “everyone.” (D) is true but weaker than (B) and drops the car-commuter link the stimulus lets you keep. (E) reverses the second statement.
3. Answer: (A). The rule is (adopted AND clay-rich) → yield increase. Farm Q adopted the method but had no increase, so the contrapositive fires: ¬increase → ¬(adopted AND clay-rich), which means not both. Since Farm Q did adopt, the part that must fail is clay-rich soil. So Farm Q does not have clay-rich soil. (B) is new information about sandy soil, never mentioned. (C) is a normative leap: “should switch” is a value judgment, and a factual stimulus cannot prove it. (D) is unsupported; the stimulus gives no count of how many adopters had clay-rich soil. (E) reverses and overstates the rule; clay-rich soil increases yield only in combination with adopting the method, and the rule does not say clay-rich soil alone guarantees anything.
A→B + ¬B ⟹ ¬A; A→B + B→C ⟹ A→C) and the quantifier overlaps from Chapter 4.Put it to work: drill Must Be True and Inference questions in the practice platform, free during early access, with every answer choice explained.