Logical Reasoning · Necessary Assumption

PrepTest 155 · Section 2 · Question 4

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 claim that money is better spent on option two than option one quietly needs option two to give more benefit for each dollar.

The basic logic

The argument shows option one gives only a small benefit, then says the real test is whether a better use exists, then names option two as better. The missing plank: option two gives more for the money than option one. Pull that plank and the conclusion falls.

Tiny words that decide it

“rather”
“only”
“instead”

Playground logic

Owen says a bigger cage barely cheers up his hamster, so his money is better spent on toys. His secret sauce: toys make the hamster happier per coin than the cage would. Without that, the switch makes no sense.

Step 2 · The kid story

Owen's Hamster Budget

Owen has saved twenty dollars for his hamster, Nibbles. Everyone at school says, "Buy a giant cage!" Owen read that bigger cages make hamsters only a tiny bit happier. "My money is limited," he tells Priya. "So the question isn't 'Will a big cage help at all?' It's 'Can I do better with my twenty dollars?' I'll spend it on chew toys and tunnels instead." Priya sips her juice box and pulls out a sticky note. "You are assuming something big. You're assuming that, coin for coin, toys make Nibbles happier than a bigger cage would. If toys did even less, your plan would flop." Owen looks at Nibbles, who is sleeping inside a cardboard tube. "Fair point," he whispers.

Step 3 · Try the model question

Bigger cages make hamsters only slightly happier. Since Owen's money is limited, the real question is whether there is a better way to spend it. So Owen should put his money toward chew toys and tunnels instead.

Owen's argument depends on which one of these assumptions?

Step 4 · The courtroom tale

The Case of the Town Fountain

A town council planned to spend its small budget on a fancy new fountain. Studies showed fountains raise park visits only a little. Mr. Whitfield argued, "Your Honor, with limited funds, the test is not whether the fountain helps at all, but whether something else helps more. The money should go to shade trees instead." The opposing lawyer stood. "Counsel, your case rests on one thing you never proved: that trees bring more visitors per dollar than the fountain." Mr. Whitfield tugged his suspenders and smiled sheepishly. "I suppose if trees did worse per dollar, my whole argument would wilt." The judge said, "Exactly. Bring me that comparison."

Step 5 · Work the real question

Open PrepTest 155, Section 2, Question 4 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Find the conclusion that one use of money beats another. Notice that the evidence mostly shows the first option gives only a small benefit. Ask what must be true for the second option to be the better pick. Use the Pull the Plank test on each choice: negate it and see whether the conclusion collapses. Be wary of choices that go further than needed or that discuss popularity or difficulty instead of value per dollar.