Logical Reasoning · Necessary Assumption

PrepTest 115 · Section 2 · Question 7

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

Step 1 · Pre-Question Guidance · read this first

Question type

Necessary Assumption

The one idea it tests

A plan that blocks a problem by filling every channel with a substitute needs the substitute not to cause the same problem.

The basic logic

The argument says trouble comes through two channels, and the remedy occupies both channels with something else. Pull the Plank: if the replacement activity could itself cause the trouble, the plan collapses. So the argument needs that the replacement is harmless in that way.

Tiny words that decide it

“solely”
“fully”
“either”

Playground logic

Tommy says his baby cousin makes messes with either hand, so giving one hand a rattle and the other a teddy will stop the mess. The secret sauce: the rattle and teddy won't make a mess themselves.

Step 2 · The kid story

Operation Busy Hands

Tommy's baby cousin Pip makes a mess with whichever hand is free: left hand grabs crackers, right hand flings peas. Tommy had a plan. "Pip makes messes only with his hands. If I give his left hand a rattle and his right hand a teddy, both hands are fully busy. No free hands, no mess!" Lulu lifted her clipboard. "One question. What if Pip shakes the rattle so hard it throws drool and crumbs everywhere? Or dunks the teddy in the applesauce?" Tommy froze. "Then the toys would make the mess." "So your plan only works if the toys themselves won't make a mess," Lulu said. Biscuit, sitting under the high chair waiting for peas, looked deeply disappointed by this whole conversation.

Step 3 · Try the model question

Tommy: Pip makes messes using either of his hands. So if Pip's only problem is free hands, giving him a rattle for his left hand and a teddy for his right will stop the messes, because both hands will be fully busy.

Tommy's plan depends on which one of these being so?

Step 4 · The courtroom tale

The Two-Door Raccoon Defense

A campground was sued because raccoons kept raiding the snack shed, which has a front door and a back door. The campground's lawyer explained the fix: "We'll park a big storage crate in front of each door. Doors fully blocked, raccoons kept out." Mr. Whitfield, for the campers, stroked his suspenders. "Your Honor, what is inside those crates?" The lawyer checked his notes. "Um... extra trail mix." The courtroom went quiet. "So the very things blocking the doors are full of raccoon snacks," Mr. Whitfield said. "The plan only works if the blockers don't attract raccoons themselves." The judge ordered the crates filled with rocks instead, and the raccoons filed an appeal, which was denied.

Step 5 · Work the real question

Open PrepTest 115, Section 2, Question 7 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Map the argument: the problem, the channels it comes through, and the remedy that occupies those channels. Then ask what the remedy quietly depends on. Predict: the filler activity must not produce the very problem it is meant to block. For each choice, Pull the Plank: negate it and see whether the plan falls apart. If the plan survives the negation, the choice is not needed, however reasonable it sounds.