Q14

[mitrakhanom1]

I picked answer A which was wrong when the correct answer is C. I'm confused as to why the answer is C. the reason why I picked A is because since T cannot graduate in 1990 and have its car purchase in 91 and T can't graduate in 91 because then the car purchase would be in 92. R and S can't graduate in 92 and have the car purchase in 91 because then T would be in front which breaks the R graduates before T rule and T's car purchase can't fit in 91 anyways.

When I looked at C, i didn't pick it because the only problem was it broke the R graduates before T rule. Whereas answer A, there is no way it will work. Please let me know what you think about my logic and where I went wrong. Thanks.

[christine.defenbaugh]

mitrakhanom1, I'm afraid I don't entirely follow your reasoning here. I think you might have turned a few of the rules around.

Let's start from the top. We have 5 possible years where things can happen: 1991-1995. In those years, 6 things have to happen: R, S, and T have to graduate and get cars. (Rc, Sc, Tc, Rg, Sg, and Tg) Multiple of these items can happen in the same year, and we are given a number of rules.

Since Sc must come before Sg (rule 3), and Sg happens in the same year as Rg (rule 4), and Rg must happen before Tg (rule 1), and Tg must happen before Tc (rule 2), we can chain these together into one sequence. This is relative ordering! Check out the diagram:


Now we need to combine this with the final rule that someone has to graduate in 1993. Since R and S graduate together, there's only two possibilities for 1993 - either R and S graduate together in that year, or T graduates in that year. Whichever one happens will have some serious consequences for everything else in the chain, so it looks like an opportunity for frames.



In Frame #1, if S and R graduate in 1993, that forces Tg into 1994, and Tc into 1995. Sc can be either 1991 or 1992, and Rc can go wherever! Similarly, in Frame #2, if T graduates in 1993, Rg and Sg must be in 1992, forcing Sc into 1991. Tc is limited to 1994 or 1995, and Rc can again go wherever!

In Frame #2, two friends (R and S) can get their cars together in 1991 or 1992. In Frame #3, two friends (R and T) can get their cars together in 1994 or 1995. It doesn't work in 1993 though! Only R can get his car in 1993. So (C) must be false! You can never have two friends get their cars together in 1993.

Does that help clear this up a bit?