powerscore lsat course book page 89 game 3 #23 December 1997

[saranash1]

I get how you can eliminate g, u, & w from question 18 but I can't figure out how to decide between M & N. I know that N would obviously be more likely because of the inferences drawn making a bunch of new rules pertaining to N but how do you know for sure?

[Jon Denning]

Hey Sara - thanks for the question. To determine if something CANNOT ever be prescribed, let's see what would happen if we actually tried to prescribe it.

First, we know we have 5 in, and 4 out. So if we have a situation where we can't meet those exact numbers, we've violated the rules. Now let's see what would occur with N prescribed:

N (dietary regimen) is prescribed, which means the other two dietary regimens--M and O--cannot be. That's two out.

If O is out, then F is also out. That's three (M O F) out so far.

V requires both H and M. We don't have M, so we cannot have V. That's four out, meaning the remaining five--N, G, H, U, W--must all be in. Where's the problem with that group?

The fifth rule tells us that if N and U are both in (and they are here), then G cannot be prescribed. So the group of five above cannot happen. What caused that issue? Prescribing N. So that means N can never be prescribed.

What about M? The group M, H, V, W, and G would work, so M is okay.

[saranash1]

Thanks! That makes perfect sense!