LSAT Necessary Assumption Practice Question: Per-Unit vs. Total
An original necessary assumption question from our free Question of the Day rotation, built around a trap the LSAT reuses constantly: per-unit vs. total. Try it first, then read the explanation — it teaches the move, not just the answer.
Your job:Find what the argument must be assuming.
Café Milo currently uses dairy milk in most of its drinks. Next month it will make oat milk — which costs the café less per liter than dairy milk — the default in every drink. The owner concludes that the café's monthly spending on milk will therefore decrease.
The owner's argument depends on assuming which one of the following?
Show the answer & full explanation
The correct answer is E
The JBDM line
Just because oat milk costs less per liter doesn't mean the total milk bill goes down.
The maybes
- MaybeMaybe baristas pour more oat milk per drink than dairy.
- MaybeMaybe the cheaper default drives up drink volume so much that milk spending rises.
Why E is right
Run the Negation Test: suppose milk use DOES increase enough to offset the cheaper price. Then spending doesn't fall, and the conclusion collapses. When negating an answer destroys the argument, that answer is necessary. Notice how soft and defensive it is — that's the NA signature.
Why the others are wrong
- AHealth has nothing to do with the cost conclusion.
- BCustomers can dislike the taste and the milk bill can still shrink — preference isn't required.
- C'Never' is far too strong; the argument only concerns next month's spending, not all time.
- DOther cafés' results aren't required for this café's math to work.
Sammy's note
Necessary assumptions feel underwhelming. Negate them — if the argument dies, you've found it.
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
Answer today's question and start a streak — then see how Sammy teaches the whole system, JBDM included, in a free lesson.