Logical reasoning PrepTest 125 · Section 2 · Question 23

Incorrect. Before we diagram, a quick note on the phrase "The only." While "only" introduces a necessary condition, the phrase "the only," through some grammatical trickery, separates the "only" from the necessary condition—the phrase that follows "the only" is sufficient. This is tricky, but think about this example to wrap your head around it—The only people who win the lottery are those who have tickets. This isn't Ticket → Win Lottery; it's Win Lottery → Ticket.
Food in Diane's apartment → In fridge
Food purchased in past week → In fridge . . . and we can eliminate this answer, as the premises share a necessary condition.

Incorrect. This answer can be eliminated after the first sentence, as it presents two conditionals (or a complex AND conditional).

Correct. Argument or Facts:
Argument

Valid or Flawed:
Valid

Question Type:
Parallel Reasoning

Stimulus Summary:
Mattress sold at Southgate Mall → Sold at Mattress Madness (where the deals are crazy!)
Sold at Mattress Madness → 20% off
Therefore - Mattress sold at Southgate Mall → 20% off

Answer Anticipation:
For a question #20, this stimulus is relatively straightforward. It presents two conditional statements that chain together, and a conclusion that's the valid representation of that chain.

Because of how straightforward the stimulus is, we're expecting the answer choices to lack easy eliminations, so once we have the generic conditional form here, we should move down quickly because we're probably going to be diagramming quite a bit!
A → B
B → C
Therefore - A → C

Answer Explanation:
Food in Diane's apartment → In fridge
In fridge → Purchased in past week
Therefore - Food in Diane's apartment → Purchased in past week
This answer matches the conditional form of the stimulus, so this is the correct answer.

Key Takeaway:
Two takeaways.

First, "the only" is followed by a sufficient condition. While "only" is still referring to the necessary condition, it's using grammatical trickery to split the conditional keyword and the condition it's modifying. It's easiest to treat "the only" as a sufficient indicator phrase.

Second, when a question later on in the section seems to have a straightforward stimulus, one of three things is true. Either you're an LSAT master and can see through the difficulty, or you're missing something and probably won't realize it until you eliminate all answers (or can't eliminate all but one), or the answer choices are going to be difficult. This question fell into the third category—relatively straightforward stimulus with answer choices that required work to eliminate.

Incorrect. Food in Diane's apartment → In fridge
In fridge → Purchased in past week
Therefore - Purchased in past week . . . and we can eliminate this answer without diagramming that last part (every second helps!). This answer likely has an illegal reversal since the sufficient condition in the conclusion is the necessary condition of a premise.

Incorrect. Food purchased in past week → In fridge
Food purchased in past week . . . the two premises share a sufficient condition, which doesn't match the stimulus.