Logical Reasoning · Flaw

PrepTest 124 · Section 2 · Question 20

Phase 1 (your first ~400 questions): read the guidance, then answer. No clock.

Step 1 · Pre-Question Guidance · read this first

Question type

Flaw

The one idea it tests

Two groups each overlap with a third group, and the argument concludes the two groups overlap with each other. They might not.

The basic logic

Draw three circles. Some of the middle group sits in circle one, and some sits in circle two. Nothing says those two slices touch. The flaw is assuming an overlap between the outer circles just because each touches the middle.

Tiny words that decide it

“some”
“therefore”
“on some”

Playground logic

Marcus's patty: some chess club kids are left-handed, and some chess club kids play violin. His top bun: some violinists are left-handed. The secret sauce, that the two slices share a kid, is missing: the left-handers and the violinists could be totally different kids.

Step 2 · The kid story

Marcus and the Chess Club Mystery

Marcus studied the chess club roster like a detective. "Clue one: some chess club kids are left-handed. Clue two: some chess club kids play violin. Conclusion: some violin players are left-handed!" Priya set down her juice box and drew three circles on a napkin. "Chess club is the big middle circle. Here are the lefties, over on this side. Here are the violin players, over on that side." She drew them far apart. "Both touch chess club. But do they touch each other?" Marcus stared. "Not in your drawing." "Not in your clues either," said Priya. Lulu checked her clipboard. The lefties were Owen and Nadia. The violinists were Kevin and Frankie. No kid was in both groups. Kevin played a dinosaur song on violin to celebrate.

Step 3 · Try the model question

Marcus: Some members of the chess club are left-handed, and some members of the chess club play the violin. So some violin players are left-handed.

Marcus's reasoning is most vulnerable to which criticism?

Step 4 · The courtroom tale

The Case of the Two Witnesses

In a fictional trial about a missing garden gnome, the prosecutor reasoned aloud. "Some people at the block party wore red hats. Some people at the block party left early. So, Your Honor, some red-hat wearers left early, and the thief wore a red hat and left early." Mr. Whitfield rose and drew three circles on an easel. "Here is the block party. Here are the red hats, on the left. Here are the early leavers, on the right. Both circles sit inside the party. Neither one has to touch the other." Judge Patel leaned forward. "Counsel, do you have even one guest who did both?" The prosecutor flipped pages. He did not. The gnome, it turned out, had been borrowed by Biscuit.

Step 5 · Work the real question

Open PrepTest 124, Section 2, Question 20 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Turn each premise into a circle diagram. Notice that both premises use the same middle group and the word some. Ask whether the two slices of that middle group must share any member. If not, the conclusion links groups that may never meet. The right answer describes that structure in general terms: two groups each overlapping a third without overlapping each other.