## Dec 2004 LG # 3 Photographs

Prepare for the LSAT or discuss it with others in this forum.
roballen

Posts: 55
Joined: Wed Jun 23, 2010 9:34 am

### Dec 2004 LG # 3 Photographs

Can someone please tell me if I diagrammed this game right and how you get answers 16 & 17

S-->W
/W-->/S

U-->S
/S-->/U

/Y-->R
/R-->Y

W-->/T or /R
T & R-->/W

so then I made this chain:

U-->S-->W--> /T or /Y-->Y

Contrapositive:

/Y-->T & R-->/W-->/S-->/U ( the Y pointing to the R)

Knock

Posts: 5151
Joined: Wed Jun 10, 2009 3:09 pm

### Re: Dec 2004 LG # 3 Photographs

roballen wrote:Can someone please tell me if I diagrammed this game right and how you get answers 16 & 17

S-->W
/W-->/S

U-->S
/S-->/U

/Y-->R
/R-->Y

W-->/T or /R
T & R-->/W

so then I made this chain:

U-->S-->W--> /T or /Y-->Y

Contrapositive:

/Y-->T & R-->/W-->/S-->/U ( the Y pointing to the R)

Just took this PT today. Let me go grab my PT booklet, and I will help you out.

Edit:

Okay, rule diagrams:
S --> W (if S is chosen, W must be chosen)
U --> S (if U is chosen, S must be chosen)
Y --> R (if Y is NOT chosen, then R must be chosen); It is important to note here that this rule is only talking about when Y is NOT selected. If Y is selected, then it doesn't apply, and so Y and R CAN go together.
W -->T + R (if W is chosen, T AND R must NOT be chosen)

We can combine the first two rules to one super rule, U --> S --> W So we know everytime U is chosen, S and W must be chosen. Everytime S is chosen, W must be chosen. Also note, that due to the fourth rule, if W is chosen, T AND R must NOT be chosen. So we also must note that when U or S are chosen, [W must be chosen] and T AND R must NOT be chosen.

Contra-positives (I don't personally write them out but it might be helpful to you):
W --> S --> U (if W is NOT chosen, S can NOT be chosen, and U can NOT be chosen; if S is NOT chosen then U can NOT be chosen).
R --> Y (if R is not selected, Y must be selected); It is important to note here that this rule is only talking about when R is NOT selected; if R is selected, it doesn't apply, ie R and Y CAN go together.
T or R --> W (if T OR R are selected, W can NOT be selected).

Okay, now that we have our base set up, let's look at the questions that were giving you trouble.

#16:
If U and Z are selected, then how many other friends MUST also appear. So we're looking for what MUST be true.

Let's see what happens when U and Z are selected. Because of our super-rule (U --> S --> W), we know that when U is selected, S and W MUST be selected. We know because of rule #4 that if W is selected, T AND R CANNOT be selected. So at this point we got:

Selected- U Z S W
Not selected- T R

So now we only have Y left to figure out. Well based on our contrapositive of rule #3, (R --> Y) we know that Y must be selected.

So at this point (the end) we have:
Selected- U Z S W Y
Not selected- T R

But remember to answer exactly what the question is asking. In this case, it is in ADDITION to U and Z how many friends MUST appear in the photograph. So we know that the friends that MUST appear besides U and Z are: S, W, and Y. So the credited answer is B) Three.

#17.
We're looking for a could be true EXCEPT (translate that to can't be true) answer that functions on having EXACTLY 3 friends selected.

This one is easier answered by going through each answer choice and thinking (or writing down) the consequences of selecting the people each answer choice have selected.

A) S and Z both are selected. Well if S is selected, we know from the rules that W must also be selected. And if W is selected, T AND R can NOT be selected. And if R is NOT selected, Y MUST be selected. This leaves us with 4 selections that must occur, S, Z, W, Y; this violates the question conditions that EXACTLY 3 people are selected. We don't have to go any further, answer choice A can't be true, and is thus correct. You can keep going and repeat the same process for B-E if you wanted to check your answer, but in the interest of time i'd move on immediately after finding A is the credited answer.

Hope this helped, and please feel free to pm me if you have any further questions.