#22 - Global, Justify

[Administrator]

Please post below with any questions!

[soobin903]

Hello, Powerscore,

As the videos have been removed, and as you posted in a separate posting that you would NOT recommend that we reach the answer by listing all nine of the solutions,
which method would you recommend for determining the virus transmission sequence?
I had narrowed down my choices to C and D, as I had ruled out A,B,E from the answer choices, but could not really determine which of the two would be the correct answer.

Thank you in advance!

[Luke Haqq]

Hi soobin903!

Great question!

First, in terms of general strategy, this question comes as the second to last in the games section. Depending on how you approach the logic games section (some people choose to do them out of order), that might mean there was little time left by the time you got to this question. Given that you note that you were down to choosing between (C) and (D), it's therefore worth noting how you might have opted for the correct answer, (C), even without diagramming anything at all.

Namely, between those two choices, look at the difference between the extent to which these variables are constrained. The question asks for which option would completely determine the others. In answer (D), the variables are U and P. These two variables show up only in one of the game's rules. By contrast, answer (C) refers to T and S, which appear in four rules combined. This strongly suggests that T and S would be much more constrained--that is, what happens to them would be more decisive as to what happens to the rest of the variables. Since this question asks for a complete determination, one might therefore have guessed (C) over (D), since the variables T and S are more decisive and connected to other variables through multiple rules.

Second, one can see more specifically why (C) is the correct answer. That answer choice states, "T transmitted the virus to S." As shorthand, we can represent this as:

T S

The game also provides us with a connected rule: "The computer that transmitted the virus to R also transmitted it to S." Since we know that T is the computer that transmitted the virus to S, we therefore know that T is also the computer that transmitted it to R. We also know that T only transmitted it to S and R, because of the rule, "No computer transmitted the virus to more than two other computers on the network." We therefore have:

T S

and
R

The game also includes a rule that "Either R or T transmitted the virus to Q." We know that T not transmit to Q, because we already know T transmitted to a max of S and R. Since it could not be T, we therefore would know that R transmitted the virus to Q. We could therefore add:

T S

and
R Q

Finally, a rule tells us that Either "T or U transmitted the virus to P." Again, since we know T transmitted only to S and R, then we know that U transmitted to P (U P). However, it might not be clear where this U P block would go. We ultimately know it must go after S because we are given the rule that "S transmitted the virus to exactly one other computer on the network" (which could not be T, R, or Q), and the rule that " Each computer received the virus exactly once" (since P already receives it from U, we know that P does not also receive it from S). This leaves us with a sole option--U is the only other computer that S transmits to, or:

T S U P

and
R Q

If we started from a given that T transmitted to S, in other words, all of the other variables would be determined.

Hope that helps!

[ange.li6778]

Hi powerscore, can you explain why E is wrong? My thought process goes like this:

U --> R --> Q
--> S --> T --> P

[katehos]

Hi Ange,

Answer choice (E) is very appealing, but it does not fully determine the spread of the virus. Your thought process is correct, but it is still possible that Q is infected by T, as opposed to R.

The infection could spread like this as well:
U R
and S T Q
                              and P

Since there is still room for a second possibility, answer choice (E) is incorrect, whereas answer choice (C) is correct since it only leaves room for one possibility.

Hope that helps!
- Kate

[bebeg3168]

Hi Ange,

Answer choice (E) is very appealing, but it does not fully determine the spread of the virus. Your thought process is correct, but it is still possible that Q is infected by T, as opposed to R.

The infection could spread like this as well:
U R
and S T Q
                              and P

Since there is still room for a second possibility, answer choice (E) is incorrect, whereas answer choice (C) is correct since it only leaves room for one possibility.

Hope that helps!
- Kate

Hi Kate,

Isn't it also true that S cannot transmit eh virus to more than 1 computer? I'm wondering if I misread the rule?

[Adam Tyson]

That is true, but in these scenarios S IS only transferring it to one computer - T. What happens after that has nothing to do with S! Once S transmits it to T, T can still transmit it to others without violating that rule. The same would be true in a case where S transmitted the virus to U; U could then transmit it to P.

It's just like contact tracing with a real virus, like Covid. If I get it, and I infect my wife, and then she goes to work and infects her coworkers, I didn't infect anyone at her office; she did!

[mollylynch]

Just to clarify:

I have-


/ R---Q
T
\ S --- P---U

So even though there are two computers after S, it is not S transmitting to U because it went through P after?

[Adam Tyson]

That's true, Molly, but S cannot transfer the virus to P, because P has to get it from either T or else U. Reverse the order of P and U in your diagram and you'll have it right (S transfers to U, U transfers to P.)

[lalalala]

Hello!

I'm not sure if my brain fog is preventing me from seeing why B is wrong. When I did the problem, both B and C were fully determined for me.

B would go like:

T -- Q -- R
-- S -- U -- P

T -- Q -- U -- S&R wouldn't work because then U/T can't transmit to P.
U -- T-- Q wouldn't work because either U/T can't transmit to P or if T goes to Q and P then there is nothing left for S to transmit to.
T--Q--P wouldn't work because Q can't transmit to P.

So since T--Q cannot then transmit to U or P, Q must transmit to R and S. That leaves only U and P. Since S must transmit to one, it transmits to U which then gives it to P.

What am I missing here! This seems completely determined!!

Please help.

You guys are the best! Thank you in advance!!