PT7.S1.Q11 - in 1990, major engine repairs

Accounts Playable

Hey guys, I'm doing some older questions as a 5th section on my PTs, and I decided to take PT 7's first LR section. I'm BRing it right now, and I can't for the life of me figure this one out; I skipped it twice during the exam, and I'm still just as clueless on it during BR.

It's a resolve/reconcile question.

In 1990, major engine repairs were done on 10% of NMC cars made in the 1970s while only 5% of those made from the 1960s had major engine repairs done.

What I am looking for: We need to explain the difference. What if cars from the 1960s had sturdier engines or something? What if NMC cranked up production in the 1970s, and cranked out a ton of cars with bad engines?

Answer A: So what? The cars have ALREADY been registered; who cares about the requirements beforehand?

Answer B: I think this sort of makes it stranger. If newer cars (1970s) are driven more carefully than older cars (1960s), then why do cars from the 1970s have a higher proportion of engine repair?

Answer C: This is the credited answer, but huh? What does scrapping the car have to do with anything? This is saying that the 1960s cars are more likely to be scrapped/not repaired than 1970s cars. I just don't see how this resolves anything or is relevant to the issue.

Answer OK, but does simplified mean easier to break? This does nothing.

Answer E: This is what I ended up picking, but I really didn't like it (I felt good enough about my POE; plus, I had to choose something since I had skipped this twice). I think this is sort of similar to the idea in answer choice A. Some of the repairs from the 1960s cars could have been avoided if the owners weren't lazy with repairs. But, so what? We are talking about cars that WERE repaired, so this fact doesn't explain anything about the figures given. Why is it still the case that the 1960s cars were repaired at a lower proportion?

LSATisland

Hi @"Accounts Playable"

The difference between 70s and 60s relates to the percentage of repairs on cars still registered.

(C) explains the difference by saying that people will be more likely to discard older cars when major engine repair is needed. So many cars that need repair will just vanish and no longer be registered. These cars are not included in the percentage. To be clear, if instead of scrapping the cars, the owners repaired them and kept registration, the percentage would be higher than 5 percent.

Accounts Playable

Thanks for the reply!

So, is the percentage given equal to (Currently registered cars that had major engine repairs/Cars manufactured that decade)?

To illustrate, does the10% mean that 10% of the NMC cars manufactured during the 1970s are still registered (as of 1990) and underwent major engine repairs.

If this is the case, I'm still not seeing how C helps resolve the problem Wouldn't scraped cars (i.e. unregistered cars) already be accounted for in the percentages? In other words, if being registered is a component of the numerator, wouldn't saying something about unregistered cars not be relevant?

LSATisland

@"Accounts Playable" said:
So, is the percentage given equal to (Currently registered cars that had major engine repairs/Cars manufactured that decade)?

The percentage is of registered cars built in the particular decade. (It is not percentage of cars "that had major engine repairs.")

(C) explains why there are more instances of repairs for the 70s cars. For example, suppose the same amount of cars need major repairs whether from the 70s or 60s. The owners of the 70s cars are more likely to go to the mechanic for the major repairs than tossing the car. The 60s owners will toss the car more than the 70s will. This will prevent more occurrences of major repairs on the 60s cars, but it doesn't mean the 60s cars are different or have less of a need for major repairs.

runiggyrun

I think the percentage is
Cars taken in for repair in 1990/Cars from the 70s (or 60s) still registered in 1990.
I believe C as a correct answer hinges on what we take "still registered" to mean. The way I read it, it's meant to signify "cars manufactured in the respective decade, excluding those deregistered/junked before 1990", so equivalent to "Cars manufactured in the respective decade and still registered at the beginning of 1990"
I think that's a reasonable way to read it, even though it's not made absolutely crystal clear. The intention of the author is to provide sort of an "apples to apples" comparison. If you don't exclude the cars already junked, of course there's going to be fewer ancient cars taken in for repairs, just because there are fewer of them on the road. After all, what percent of cars manufactured in 1910 still get taken in for repairs in 2016? And you're not going to do percentages based on "cars that were registered at the end of 1990" because you might end up with >100% of the cars repaired.
With that tiny assumption in mind, it makes sense that C will be the correct answer (and a reconcile answer doesn't have to be ironclad, like an MBT).
Let's say we have 2000 cars from the 70's and 1000 cars from the 60's still registered at the beginning of 1990. 210 of the 70's break down, and 200 get repaired (10%). The other 10 get junked.
Now, 500 of the 60's cars break down, but only 50 get repaired. The other 450 get junked because they are old and crappy and who wants to pay $3000 for a transmission on a car that can be sold for $500 AFTER you fix it?

ChiTownGuy

My thing about C, even though it jumped out at me at first was that no where in the stimulus does it say that the major engine repairs were needed "to keep the car in operation". Which is why I then chose D, because it's like if the engine on the 70s cars are simpler, than it makes sense more people would have them repaired, it probably costs less. Think labor costs, they charge you more for more complicated work. So, I guess I'm not really sure why C is correct, when you have to assume that the major engine repairs were needed to keep the car in operation. What if it was like major repairs to fix the emissions, but it still ran or something idk. Like if the stimulus would have said something that indicated that the repairs were needed to keep it in operations, or if the choice would have said something like "Cars from this particular manufacturer needed major engine repair to keep them operating, and blah blah blah about the likely hood of scap when that is the case" then ok, but it didn't. This is outrageous.