Question 21 is a global question, and it's asking any of the following computers could have transmitted the virus to two other computers, except. So the four wrong answers here could be a computer that transmits to two other computers. The correct answer is something that cannot be the computer or rather be a computer that transmits to two other ones.

The correct answer is not gonna be allowed to go in this spot. Now one of the first things I would do with this question is to look back at any prior work that I have, to see if I have examples of letters transmitting to two other computers. But if you don't have that prior work, then you'll just have to assess these answers and see, could they be a computer that transmits to two others?

So A says P, and could you have P transmit to two others, for example, P going to R and S? I don't see why not, because you could just have T go to P like this to satisfy rule five. You could have R go to Q to satisfy rule four, and then you could have S go to U to satisfy rule two since S has to transmit to somebody.

So P could transmit to others. What about Q, could Q transmit to two others? Well, let's put the Q over here and see if that works. Now I'll have the T go to Q to satisfy rule number 4. I'll have the T go to P to satisfy rule number 5. And the final letter left is the U, I could just put the U over here to satisfy rule number 2, and this works.

And as a bonus, I've just proven that Q can transmit to two, and T can also transmit to two different computers. So that's gonna get rid of answer choice B and D. What about R here, could R transmit to two others? So here's the R, if R is going to transmit to two others, it's got to look like this. I need to fill something over here, over here, I still need something over here, and over here.

So let's try to make this work, rule number 4 says Q gets it from R or T, so let's just have R pass to Q like this. Number 5 says P gets a virus from T or U, so that means P can't be getting it from R here, P can't be getting it from S, P can't get it from Q. So that means P would have to be over here, and then it would have to get the virus from either T or U.

But now we're running into a problem here because we already have the Q down, the R, the S, and the P, so that's four letters so far over here. A fifth letter is gonna have to go over here cuz it's either gona be T or U. So that means we were only going to be left with T or U, one of T or U to place over here, and over here, that's not going to work. And you can count up the spaces, one, two, three, four, five, six, seven.

That doesn't work because you only have six different computers. So this is why R actually cannot transmit to two other computers. Now this is a difficult question, so I wanna run through that logic one more time. If you try to make R pass to two other computers, then right now we have to fill in one, two, three, four open spaces.

The R and the S are already set in our virus order, and that means the other four letters have to occupy these blue circles. But rule number 5 says that P has to get the virus from either T or U. And that's gonna rule out P from all of these circles over here and force it into the first spot. But now if you try to make T or U go before P, you're not gonna have three more computers to go in these three circles.

So that's why R is the correct answer to our except question. Now we can also prove that you could transmit to two, you could put the U over here. Let's make the R go to Q to satisfy rule number 4. And let's make the T go to P to satisfy rule number 5. That means the T would have to go over here cuz S has to transmit to somebody, and you could put the P after like this.

This is something that satisfies all the rules, and that proves you could transmit to two different computers.