Logical Reasoning · Parallel Flaw

PrepTest 104 · Section 4 · Question 13

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

Step 1 · Pre-Question Guidance · read this first

Question type

Parallel Flaw

The one idea it tests

What is true of each member is not automatically true of the group they form.

The basic logic

X-Ray the argument: a property belongs (or does not belong) to every individual, so the argument hands the same property to the whole collection. That is a Team-to-Star Mix-Up running from parts to whole. Find the choice that moves a property from each member to the group in that same direction.

Tiny words that decide it

“so”
“just a collection”
“no less”

Playground logic

Marcus says no single fifth grader can lift the stage piano, so the fifth grade as a class cannot move it either. He took a fact about each kid and slapped it on the whole group. The X-Ray shows: parts lack it, so the whole lacks it.

Step 2 · The kid story

The Piano Problem

The spring show needed the old stage piano moved to the gym. Ms. Rivera asked the fifth grade for help. Marcus put his hands on his hips. "Impossible. Nadia can't lift it. Owen can't lift it. I tried, and I can't lift it. Nobody in our class can lift that piano. So our class can't move it." Priya set down her juice box, walked over, and counted. "One, two, three..." Twelve fifth graders grabbed a side, and the piano rolled down the hall like a parade float. Marcus jogged after them. "Okay, but I was right about each of us." Lulu wrote on her clipboard: what each one cannot do, all of them together might.

Step 3 · Try the model question

Marcus: No one in our class can lift the stage piano. Our class is just a group of those kids, so our class cannot move the piano either.

Which one of the following contains flawed reasoning most like Marcus's?

Step 4 · The courtroom tale

The Case of the Heavy Boat

Ms. Pike argued for the rowing club. "Your Honor, not one rower in the club can drag the club's big boat out of the water alone. The club is only a collection of rowers. Therefore the club cannot drag the boat out, and it cannot be blamed for leaving it on the town dock." Mr. Whitfield tugged his suspenders. "Your Honor, I watched eight of those rowers carry it in last May. Took them two minutes." Judge Marlow nodded. "Ms. Pike, what no single member can do, a group of members very often can. The club will move its boat by Friday."

Step 5 · Work the real question

Open PrepTest 104, Section 4, Question 13 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

X-Ray the original first: a property belongs to each individual, so the argument gives the same property to the group made of them. Write that skeleton down. Then test each choice: does it move a property from every member to the whole in the same direction? Watch for choices that run from whole to part, that are valid, or that commit a different flaw on a similar topic. Only the matching skeleton wins.