Free Courses › Logical Reasoning

The big question types · Lesson 6 of 14

Sufficient assumption questions

After this lesson, you can identify a sufficient assumption question from its stem, break the argument into premise, conclusion, and gap, and pick the answer that guarantees the conclusion follows with certainty.

24 min video, plus notes and 1 practice question below

In this lesson

• Spot a sufficient assumption stem on sight, so you stop reading it like a strengthen question
• Split any argument into premise, conclusion, and gap in seconds
• Predict the missing assumption before you touch the answer choices, so wrong answers stop tempting you
• Treat strong, absolute wording like "all," "every," and "only if" as a good sign here, not a warning
• Catch the contrapositive disguise when your prediction doesn't show up word for word

What this question type is actually asking

Sufficient assumption questions ask for the one missing piece that guarantees the conclusion, not one that makes it more likely. If the answer is true, the conclusion has to follow, with zero wiggle room. That's a different job than strengthening an argument, which just makes the conclusion a bit more believable without locking it in.

Think of the argument as a broken equation. You're not picking the number that nudges the result closer to right. You're picking the exact variable that makes both sides equal. The answer is called "sufficient" because once it's true, the argument is guaranteed valid. No probability, no weighing evidence, just one test: if this were true, would the conclusion be forced to follow?

Spot the question type before you touch the argument

The question stem tells you what you're dealing with, if you know the words to watch for. Sufficient assumption stems use language like "allows," "enables," "follows logically if assumed," and "properly inferred if assumed." Those are demanding words. They're asking which single answer makes the argument airtight.

Compare that to a strengthen stem: "which one, if true, most strengthens the argument" or "provides the most support." Strengthen is open ended, about raising confidence. Sufficient assumption is binary: the answer either completes the logic or it doesn't. Once you tag the question correctly, strong or absolute wording in a choice stops looking like a trap and starts looking like a requirement.

Break the argument into premise, conclusion, and gap

Every sufficient assumption question hands you an incomplete argument. Find the premise, the evidence given as fact, and the conclusion, the claim the author wants you to accept, usually flagged by words like "therefore," "thus," "it follows that," "hence," or "clearly." Then look for what's missing between them: the gap.

Take this example. Premise: every student in the math club enjoys solving puzzles. Conclusion: Lucas enjoys solving puzzles. The argument only works if Lucas is in the math club, but nothing says that. That's the gap, and "Lucas is in the math club" is the assumption that closes it.

Sometimes the gap is a category shift instead of one missing fact about a person. Premise: all members of the ethics committee are required to attend the retreat. Conclusion: Dr. Lee is required to attend the retreat. The argument never states that Dr. Lee sits on the ethics committee, so that membership is the missing link.

Predict the missing link before you look at the choices

Before reading any answer, ask what must be true for the conclusion to follow from the premise with total certainty, not what might help or what makes the argument stronger. Take this example: premise, if a person exercises regularly, they are healthy; conclusion, Sophia is healthy. The prediction is "Sophia exercises regularly." That's the one statement that guarantees the conclusion.

Predicting first matters because the answer choices are written full of language that sounds related but doesn't actually connect the premise to the conclusion. When you already know what you're looking for, you stop asking "does this sound reasonable" and start asking "does this match what I already decided is missing."

Test each answer like a lawyer, not a reader

For every choice, ask does it bridge the gap completely. It doesn't need to sound impressive, it needs to connect the dots with nothing left open. Then ask if it's strong enough: words like "all," "every," and "only if" aren't red flags here, they're often required, because soft words like "usually" or "might" leave room for doubt.

Watch for 2 traps. First, an answer that introduces a brand new idea the stimulus never mentioned, like firefighters showing up in an argument about astronauts. Second, an answer that reverses the logic, turning "all A's are B's" into "all C's are B's" instead of connecting A to B to C. Either one gets eliminated immediately.

Finally, check whether the choice matches your prediction directly or matches its contrapositive. If your prediction was "if healthy, then exercises regularly," the correct answer might instead read "if someone does not exercise regularly, they are not healthy," the same logic flipped around. If none of the choices match your prediction word for word, flip your prediction into its contrapositive and scan again.

The short version

• If you catch yourself reading a sufficient assumption stem the way you'd read a strengthen stem, that's a sign you rushed it. Reread the stem before you touch the argument.
• Don't start with the answer choices. Circle the premise and conclusion first, write a one-line prediction, then look for the choice that matches it.
• Log it when you almost pick "none of these" on a sufficient assumption question. More often than not, your prediction is hiding in the contrapositive you didn't flip first.

Mark lesson 6 complete