## PrepTest 79, Game 4, Question 18

### Transcript

This is the first question in game number four of PT 79, and it's question number 18. It's a global question asking what could be one possible route of the virus from the first computer to Q. And you can see in the answer choices we're getting potential orders from the first computer to Q.

One of these is gonna be possible, four of them are gonna violate one or more of our rules. So, this kind of question is similar to the orientation question that we usually get as the first question of any game. And I wanna solve it similarly, I wanna see if I can use the rules to eliminate answers.

So, let's start with some rules that are easy to apply. For example, the third rule here says that there's a particular computer that's gonna pass to both R and S. And so, that means that R can't pass to S, and S can't pass to R because they'd get the virus from a different computer. That means that answer choice B is wrong because there you see S passing to R, but S isn't going to pass to R.

Now, let's take a look at the fourth rule, which tells us that Q has to get the virus from either R or T. So, that's gonna get rid of answer choice C, where Q is getting the virus from U. That violates the fourth rule. Take a look at the fifth rule. P has to get the virus from either T or U.

So, that's gonna get rid of answer choice A where it has R going to P instead of T or U going to P? And now, we have to think about answer choices D and E. Do you notice that with answer choice E it's saying that U is gonna pass to T to P to R and then to Q? So, there's only one more letter left that we have to account for in answer choice E, and that's gonna be the S.

Based on rule number three, we know that whatever passes to R also passes to S. So, in answer choice E, what's gonna happen is, because the P goes to R, the P also has to pass to S. But is this going to be consistent with all of our rules? Unfortunately, this does not allow for S to pass to exactly one other computer. We still have to account for S having exactly one transmission.

And over here, we have one, two, three, four, five, six computers already accounted for. The S is not allowed to pass to Q like that because Q is already getting the virus from R, and Q can't get the virus from both R and S. So, that's why answer choice E does not work. It does not allow S to transmit to anyone else.

Through process of elimination, that's how you could pick answer choice D. Now, if you wanna test answer choice D, that's something you could as well. D is saying U goes to P, P goes to R, and then R goes to Q. Now, since whoever passes to R also has to pass to S, that means P is gonna go to S. And since S has to transmit to exactly one other, we need something to go after the S here, and the only letter that's left is our T.

And this is an order that works which proves that D could be true.