September 2016 LSAT
Section 3
Question 18

# One possible route of the virus from the first computer in the network infected to Q is

Replies

Victoria on August 8, 2020

Hi @zia305,Happy to help explain the game!

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Now onto the game.

We know that there are six computers on a network: P, Q, R, S, T, and U.

Exactly one of these computers was infected by a virus which then spread throughout the network. Each computer received the virus exactly one time.

Rule 1 - No computer transmitted the virus to more than two other computers.

Therefore, we have a bit of a strange sequencing game. It is possible that a computer transmitted the virus to 0, 1, or 2 other computers.

Rule 2 - S transmitted the virus to exactly one other computer

Rule 3 - The computer that transmitted the virus to R also transmitted it to S.

Therefore, this computer transmitted the virus to both R and S and no other computers.

Rule 4 - Either R or T transmitted the virus to Q.

Rule 5 - Either T or U transmitted the virus to P.

Now we have a sort of skeleton diagram without including Rules 4 and 5.

S --> _

_ -->

R

There are too many possible diagrams which arise from this and the rules. A good place to start is to determine what our possible options are for the first computer to have received the virus.

We already know that S and R received the virus from another computer. We also know that Q and P received the virus from another computer. Therefore, only T or U could be the first computer.

This is a really tough game to diagram because there are so many possible options. This is an example of a logic game where it is easier to go through the answer choices and determine the correct one using a process of elimination based on the rules and the skeleton diagram we have above.

We can start by eliminating answer choice (A) because we know that only T or U could be the first computer.

Answer choice (B) is incorrect because of Rule 3. We know that the computer which transmitted the virus to S also transmitted the virus to R. Therefore, S cannot transmit the virus to R because each computer as the stimulus tells us that each computer received the virus exactly once.

Answer choice (C) is incorrect because of Rule 4. U cannot transmit to Q because Q must receive the virus from either R or T.

Answer choice (E) is incorrect because of Rules 2 and 3 and is a bit more complicated.

Rule 2 - We know that S must transmit the virus to exactly one other computer. In this answer choice, all computers have received the virus except U which means that S must be first. But we know that S cannot be first because of Rule 3.

Rule 3 - We know that S must have received the virus from the same computer which transmitted the virus to R. This means that S must have received the virus from P.

These two rules contradict one another in this setup as S must be in two different places at once.

Therefore, answer choice (D) should be our correct answer, but let's double check.

P transmitted the virus to R; therefore, P must also have transmitted the virus to S. This leaves only one computer left for S to transfer to: T.

--> R --> Q

U --> P

--> S --> T

This setup meets all of our rules.

Rule 1 - P transmitted the virus to two computers and all other computers transmitted it to one or zero.

Rule 2 - S transmitted the virus to exactly one other computer: T

Rule 3 - P transmitted the virus to both R and S

Rule 4 - R transmitted the virus to Q

Rule 5 - U transmitted the virus to P

Definitely a tough game and an unusual one at that! If you run into something similar again, focus on creating a diagram as best you can and then work through the answers using a process of elimination based on the rules.

Hope this helps! Keep up the good work and please let us know if you have any further questions.

zia305 on August 11, 2020

@Victoria Thanks So Much!!!! I really appreciate your help!