PrepTest 101, Section 3, Question 19
Every student who walks to school goes home(16 more words not shown)
The conclusion of the argument(10 more words not shown)
- ASome students who(9 more words not shown)
- BEvery student who(8 more words not shown)
- CSome students who(11 more words not shown)
- DSome students who(9 more words not shown)
- EEvery student who(7 more words not shown)
Reading the argument
Where did the part-time jobs come from? The only fact we've been given concerns walking to school and going home for lunch. It says nothing about anyone's job.
Start with what that fact does tell us. If a student walks to school, that student goes home for lunch. So a student who doesn't go home for lunch cannot be one of the walkers. Otherwise, we'd have a walker who fails to do what every walker does.
That gives us a way to reach the conclusion. Find a student with a part-time job who doesn't go home for lunch. That student would have to be a nonwalker, and we'd have exactly what the conclusion promises: at least one student with a job who doesn't walk.
What we’re looking for
The question asks what added fact would make the argument work. Our prediction is that some students with part-time jobs don't go home for lunch. We don't need every working student to stay away at lunchtime; one is enough.
Working the answers
- A
These students don't have part-time jobs, so they can't establish the conclusion about students who do. Going home for lunch also doesn't tell us whether they walk. All walkers go home, but students who take the bus might go home too.
- B
This connects jobs to lunch, but it doesn't give us any jobholders who don't walk. Imagine every student walks to school, goes home for lunch, and has a part-time job. Both the original fact and B would be true, yet the conclusion would be false: there would be no working students who don't walk. So B doesn't finish the proof.
- C
We can establish that these students don't walk: they don't go home for lunch, and every walker does. But they're students without part-time jobs. Finding nonwalkers in that group doesn't establish that there are any in the group with jobs.
- DCorrect
This gives us exactly the overlap we predicted: students who have part-time jobs and don't go home for lunch. Take any one of them. Could that student walk to school? No—every walker goes home for lunch, and this student doesn't. So at least one student with a part-time job does not walk to school, which proves the conclusion. The answer names the lunch group first, but it describes the same people as our prediction.
- E
This would add a new fact about students who go home for lunch: they all walk. We're allowed to add a new fact, so the problem isn't merely that the original statement didn't tell us this. The problem is that E still says nothing about part-time jobs. We have no way to establish a working student who doesn't walk.
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