June 2019 LSAT Section 4 Question 21

The assignment of volunteers to days is completely determined if which one of the following is true?

filozinni on August 20, 2020

Why is C correct

Could anyone please show why C is correct? How does placing M (and thus L) on Friday complete the assignment of all places? Thank you very much!

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shunhe on August 21, 2020

Hi @filozinni,

Thanks for the question! So we’re trying to completely figure out the assignment of volunteers to days here. Well, let’s put M on Friday and see what happens!

So we put M on Friday. Well, that means L works on Friday. And we already know from rule 3 that N works on Friday. So Friday is set, since it’s LMN.

Now let’s think about Saturday. What do we know about Saturday? It’s not a rule, but we know that L has to work on Saturday. Why? Because we know that P doesn’t work on Saturday, which means 3 out of 4 of LMNQ work on Saturday. Is it possible to not pick L? Well, if we don’t pick L, that means we pick MNQ. But if we pick M, we have to pick L! So it’s impossible to not pick L, so L always goes on Saturday. So L is on Saturday and Friday, which means from rule 1 that L can’t work on Thursday.

OK, now pause, let’s think about Thursday. If L can’t work on Thursday, that means M can’t work on Thursday (by the contrapositive of the second rule). But that means three people are left for Thursday: NPQ. So Thursday is set now too.

Now back to Saturday: if N is on Thursday and Friday, then N can’t be on Saturday (from rule 1). And again, we know that P doesn’t work on Saturday. So now we’re down to three again: LMQ. And that decides Saturday, which means we’re good on all days! So putting M on Friday determines everyone.

Hope this helps! Feel free to ask any other questions that you might have.

filozinni on August 21, 2020

This explanation helped me a lot! I had not made the - quite big - deduction that L is always on Saturday, and that's why I was so confused. Thanks as always for the great explanation, @Shunhe!

shunhe on August 21, 2020

You're welcome! That's a pretty important one for this game and helps a lot, but it's hard to catch at first.

Gabe85 on September 11, 2020

Any advice on learning how to make deductions like this? I keep practicing but can't seem to ever make these crucial deductions that save enough time to finish the section.

PicolinaBallerina on June 18 at 11:03PM

Hello! Do we mean B is correct here? The real time explanation picks answer choice B, and that explanation sounds like B as well?

Emil-Kunkin on June 20 at 08:39PM

Yep it is B, looks like the initial question mistyped C instead.