LSAT Flaw Practice Question: Necessary vs. Sufficient
An original flaw question from our free Question of the Day rotation, built around a trap the LSAT reuses constantly: necessary vs. sufficient. Try it first, then read the explanation — it teaches the move, not just the answer.
Your job:Name what's wrong with the argument.
Every crew that has ever won the Kestrel Cup sailing final finished in the top five of at least one qualifying regatta earlier that season. The Marlin Racing crew finished second at the Bayview qualifying regatta last weekend. Clearly, then, the Marlin crew will win this year's Kestrel Cup.
The reasoning in the argument is most vulnerable to criticism on the grounds that the argument
Show the answer & full explanation
The correct answer is A
The JBDM line
Just because every winner finished top five in a qualifier doesn't mean every top-five finisher wins.
The maybes
- MaybeMaybe dozens of crews finished top five in qualifiers, and only one can win.
- MaybeMaybe Marlin qualifies but faces far stronger crews in the final.
Why A is right
A top-five qualifying finish is a necessary condition for winning — something every winner has. The argument treats it as sufficient: have it, and you win. That's the classic necessary/sufficient flip. Lots of crews meet the requirement; at most one lifts the Cup.
Why the others are wrong
- BNobody's credibility gets attacked anywhere in this argument.
- CThere's no cause-and-effect claim here — the mistake is about conditions, not causation.
- DThe evidence and the conclusion are both about the Marlin crew.
- ENo authority is cited at all, let alone one outside the field.
Sammy's note
Necessary gets you in the door; it doesn't hand you the trophy.
What's a JBDM line?
Just Because Doesn't Mean (JBDM) is the Burton Method's foundational framework for attacking LSAT arguments: identify the premise, identify the conclusion, then say "just because [premise] doesn't mean [conclusion]" and generate "maybes" — alternative explanations that expose the gap between them. That gap is what weaken, flaw, strengthen, and assumption questions all test, so every explanation on this page starts with the JBDM line. It's a micro-lesson in the system, not just an answer key.
One question a day builds the habit
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