If Kate is scheduled for Friday, which one of the following must be true?

ishadoshi August 19, 2019

Must be true questions

What is a good way to approach must be true questions? I usually take each answer choice and then see if something else other than what the answer choice states is possible to eliminate it. For eg, if the answer choice states that Henry must go on Wed then I try out whether Henry can go on Tue, and if he can then I know that that doesn't have to be true. But as you may figure, this is very time consuming esp if the answer choice is E. I couldn't decipher one specific way to approach must be true questions in a quicker way from the video explanations on carious games - sometimes it uses previous work done on previous questions of the games, and sometimes it boils down the game to 2 scenarios and then come up with the answer. I don't however use the exact same technique that is shown in the video explanations and so may not be able to do it as you guys do - any advice on a good time saving way to approach must be true questions then?

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Irina August 19, 2019

@ishadoshi,

Must be true questions usually require you to make inferences based on the initial rules. For example for this game, we have the following initial setup:

H I K N V
___ ___ ___ ___ ___
M T W Th F
~K ~V
~N

(1) ~N (M)
(2) H (M) -> N (F)
~N(F) -> ~H(M)
(3) N(T) -> I(M)
~I(M) -> ~N(T)
(4) VK

The question gives us an additional condition - if K is scheduled for Friday, which of the following must be true. One quick way to determine the answer is to plug this condition into the initial setup and see what inferences we can make :

V K
___ ___ ___ ___ ___
M T W Th F
~K /H /H ~N
/N /N

Per rule (4), we know that VK must go together, thus V is on Th.
Per rule (2), we know that H is not scheduled for M, and must be scheduled either on T or W.
Per rule (1), we know that N cannot be scheduled for M, so N also must be scheduled either on T or W.
Since both H & N must go on T or W or W or T, there is only one contestant remaining that could go on M, and it is I. We can thus conclude that Iris must go on Monday (choice (C)).

The bottom line is the tryout approach may work for could be true questions but is a poor fit for must be true questions that can usually be answered by making inferences from the initial rules, so spending more time on the initial setup might be one strategy to try.

Let me know if you have any further questions.

ishadoshi August 20, 2019

thanks Irina!

Ravi August 20, 2019

@ishadoshi, let us know if you have any other questions!