Q21

[ShehryarB30]

Could you pls explain this

[ohthatpatrick]

RULES:
3 - 7 - 5
2 - 8
4: Thu or Fri
No consecutives together
1, 6: floaters

A "fully determined / completely determined" question usually means a lot of plug-n-chug.
We test each answer by seeing if the initial fact keeps triggering more and more domino reactions until everyone is locked into position. If the initial fact stops triggering more things, then we move onto another answer.

We can sometimes strategize by looking for an answer that pins down a floater or a reversible chunk. Or we can look for answers which offer a fact that we know would trigger something. Or we can just suck it up and get to work with plug and chug.

(A) __ __ | __ __ | __ __ | 1 __
Does this trigger anything? Doesn't seem like it. 3 - 7 -5 and 2 - 8 still have lots of options.

(B) __ __ | __ __ | 2 __ | __ __
The 2 - 8 triggers 8 on Sat: __ __ | __ __ | 2 __ | 8 __
The 3 - 7 - 5 could still go multiple ways, though.

(C) __ __ | 6 __ | __ __ | __ __
Does this trigger anything? Doesn't seem like it.

(D) __ __ | __ __ | 7 __ | __ __
The 3 - 7 - 5 puts 5 on Sat: __ __ | __ __ | 7 __ | 5 __
The 2 - 8 seems to still have options. 8 could be Thu or Sat.

(E) __ __ | __ __ | 8 __ | __ __
The 3 - 7 - 5 puts 7 on Thu, since it can't share Fri with 8.
That forces 3 on Wed: 3 __ | 7 __ | 8 __ | __ __
How about 2 - 8?
2 can't go on Wed with 3, so it'll have to be on Thurs:
3 __ | 7 2 | 8 __ | __ __

This means that 4 has to go on Fri (since it's Thu or Fri)
3 __ | 7 2 | 8 4 | __ __

3 - 7 - 5 forces 5 to be on Sat.
3 __ | 7 2 | 8 4 | 5 __

The floaters 1 and 6 are left, and 6 can't share Sat with 5, so it has to go on Wed.
3 6 | 7 2 | 8 4 | 5 1

(E) is correct. That was a DOOZY!

Remember to do your "If" questions (including these fully-determined ones) before you do your Unconditional questions, so that you can potentially get extra use out of all these scenarios we have to write.

[JeffM728]

Patrick,

Your entire explanation seems to really off. E specifically states that "The eighth floor is examined on Thursday", yet you've placed it in Friday.

[RuthS478]

Just bumping this question! It seems like you might have a different version of 21 in your system possibly? Because for the LSAC website, option C is "The fifth floor is examined on Friday" and same as what the student said above about the eigth floor option.

[Laura Damone]

Sorry about that! I'm not sure what answer set Patrick was looking at!

Definitely it's a plug and chug question, though.

A) __ __ | __ __ | 2 __ | __ __

This forces 8 into Sat but nothing else, really.

B) __ __ | 3 __ | __ __ | __ __

This forces 7 and 5 into fri and sat, respectively. It also forces 4 into Fri, since the floors in thurs can't be consecutive. For the same reason, 2 has to go wed. That leaves 1, 6 and 8 left to place. 1 and 8 can't go on wed, so 6 goes on wed with 2. But 8 and 1 can't be pinned down.

C) __ __ | __ __ | 5 __ | __ __

This forces 3 and 7 into wed and thurs, respectively. It also forces 4 into thurs, since the floors in fri can't be consecutive. Since 2 can't go on wed with 3, it has to go on fri, forcing 8 into sat. But 6 and 1, the floaters, can't be pinned down.

D) I didn't test this one because 6 is a floater. I didn't eliminate it - a floater could totally be the right answer. In fact, sometimes they are because they're otherwise hard to pin down. But It's hard to test because I didn't see a domino effect, so I decided to test E instead. If it was wrong, I would have selected D without testing.

E) __ __ | 8 __ | __ __ | __ __

This forces 2 into wed. Because 2 and 3 can't both be wed, the 3 - 7- 5 block must go thurs, fri, sat. Since Thurs is now full, 4 must go fri. That leaves 1 and 6 to place. 1 can't go with 2 on wed, so 6 must. And 6 can't go with 5 on sat, so 5 must. Thus, E completely determines the order of the elements. We have a winner!