Q20

[mturner]

Can someone please explain the thought process for questions 20 and 23? I created at least 4 different scenarios and still couldn't figure out the answers. I had a couple of dual options and could argue that each answer could be true.

[bbirdwell]

Seems like you missed an inference somewhere. The original constraints don't tell us a whole lot, but there's one important inference we need to catch: if there must be at least one space between F and G, and K is in 2... what does that mean for F? It's got to be in one!

You can symbolize it this way: F _ G, and then, K in 2, see where you can fit the F _ G piece, knowing that the G can be even further away if necessary ("at least one is between them"). There's only way to do that, and it's to put F in 1.

So the order of pilots looks like this, with four open spaces for the co-pilots below (formatting is difficult here, so I'll just explain):
F K G/L G/L

The only other constraint we have says u must go with L. Awesome!

#20 says "u -- r," so let's take a look at what impact this has on our diagram before looking at scenarios and choices. The automatic inference we should make from a constraint like this is that u cannot last and r cannot go first. How does this affect the other elements?

Well, if u cannot go fourth, L cannot go fourth, either, because L must go with u. And if L cannot go fourth, then L must go third. Thus, u goes third, and G goes fourth. And if u goes third, then r must go fourth. So now we have something like this:
F K L G
___ u r

Now, we can go to the choices and see which one can NEVER happen. It's (C)! K can go simultaneously with t, but can never go earlier.

#23 says "L -- s." We'll follow the same process, following the chain of inferences before going to the choices. It works out in a very similar fashion as #20. Since L cannot be last, it must be third, and since s must come after it, s must be fourth. We also know that u must be third, since it must be matched with L.

F K L G
___ u s

We want the choice that "could be false," which is to say that four of the choices must be true, and one of them could be but doesn't have to be. (D) is the only one that does not HAVE to be true.

Does that clear things up?