September 2016 LSAT Section 3 Question 22

# The spread of the virus among the computers is completely determined if which one of the following is true?

2 Replies

Skylar on December 15 at 01:50AM

@sprozes, happy to help!First, let's remember what we can deduce from the setup of the game.

- We know that S must transmit to exactly one other computer. It can't transmit the virus to itself, it can't transmit it to R since it receives the virus from the same computer that R does, it can't transmit it to Q because of Rule 4, and it can't transmit it to P because of Rule 4. This means that S can only transmit the virus to either T or U, as these are the only remaining options.

- T and U are also the only options that can start the chain, as R, S, Q, and P all have rules that specify how they receive the virus from other computers. Since they're receiving the virus from other computers in the network, they cannot be the ones to start it.

Now, let's look at answer choices A , B, D, and E. For each, let's see if we can come up with more than one valid scenario (for time purposes, we don't need to come up with all possible scenarios, just at least two).

A: (T/U) - R - Q

- S - (T/U) - P

or

(T/U) - P - R - Q

- S - (T/U)

B: U - R -

- S - T - Q

- P

or

T - Q - R

- S - U - P

D: U - P - R - Q

- S - T

or

T - R - Q

- S - U - P

E: U - R - Q

- S - T - P

or

U - R

- S - T - P

- Q

Since we could come up with more than one valid scenario for each of these, they do not fully determine the spread of the virus. Let's look at (C).

C: T - R - Q

- S - U - P

This is the only possible scenario when T transmits to S, so (C) is correct.

Does that make sense? Please let us know if you have additional questions!

Skylar on December 21 at 09:08PM

@sprozes, allow me to add to this briefly to explain how I derived the scenario for (C).(C) says that T transmitted the virus to S. Rule #3 says that the computer that transmitted the virus to R also transmitted it to S. Therefore, T has to transmit the virus to both S and R.

From our setup, we also know that either T or U must start the chain, and the other must receive the virus from S. Since T is transmitting the virus to S, it cannot receive it. So, T must start the chain and S must transmit the virus to U. This gives us:

- R

T - S - U

Now all we have left is placing Q and R. Rule #4 says that either R or T must transmit to Q. Since T is already transmitting to 2 other computers and Rule #1 says 2 transmissions per computer is the max, R must transmit to Q.

Rule #4 also says that either T or U must transmit to P. Since T is already transmitting to 2 other computers and Rule #1 says 2 transmissions per computer is the max, U must transmit to P. This gives us:

- R - Q

T - S - U - P

Each of the 6 variables are determined in this scenario, meaning that there are no options to rearrange them.

Hope that helps!