CR Strategy Guide (Third Edition) - Chapter 5 - Draw a Concl

[jay.mathew]

"In certain congested urban areas, commuters who use public transportation options such as trains and subways spend approximately 25 percent less time in transit, on average, to reach their destinations than commuters who opt instead to take cars to their destinations. Even individuals who drive their entire commute in carpool lanes, which are typically the least congested sections of roadways, still spend more time, on average, than commuters who use trains and subways."

When I read this, I am inclined to think that commuters who use public transport spend on average approximately 25 percent less time in transit than commuters who choose to take cars. Maybe my thinking has somewhat been contaminated by the FDP booklet, as I have just completed a few problems on percentage change, but I was thinking that this premise could be represented as follows:

Public transport transit time = approx 0.75 cars transit time.

So we don't know what the actual transit times are, but we know that public transport transit time will always be about 75% of car transport times.

The statements above, if true, best support which of the following assertions about commuting in the congested urban areas mentioned above?

(A) Waiting in traffic accounts for approximately 25 percent of the commuting time for individuals who drive to their destinations.
(B) Walking between a subway or train station and one's final destination does not, on average, take longer than walking between one's car and one's final destination.
(C) Using carpool lanes does not, on average, reduce driving time by more than 25 percent.
(D) Individuals who commute via public buses spend approximately 25 percent more time in transit than those who commute using public trains or subways.
(E) Subways and trains are available in the majority of congested urban areas.

The official answer is (C). However, I'm having trouble understanding how the premises in the passage support the assertion that "Using carpool lanes does not, on average, reduce driving time by more than 25 percent". The way I see it, carpool lanes could reduce driving time by more than 25 percent, but that kind of information is not mentioned in the passage. We only have information that tells us that public transport transit time is approximately 75% of the car transit time.

Can somebody please show me where I am going wrong?

[Ben Ku]

In this Find the Conclusion question, we can use numbers so you don't have to worry about fractions and decimals. So if it takes 100 minutes to drive to work, it should only take 75 minutes by public transportation.

The second sentence in the argument states that it still takes longer to drive using the carpool lane than to take public transportation; that means, taking the carpool lane does not reduce the transit time by more than 25 minutes. This is the conclusion drawn in (C).

(A) is an additional premise, not a conclusion, because waiting in traffic is not discussed in the argument.
(B) is describing an assumption, not a conclusion, because the time walking to/between sub/train is not discussed.
(D) is also an additional premise that may go against the other premises in the argument; however, because the argument highlights specifically subs and trains, not busses, this is not a conclusion.
(E) is irrelevant because we're not talking about the feasibility of taking subs, just the commuting time.

Hope that helps!

[shwetha_shyam]

Dont agree with above explanation.

C might not necessarily be true.

There are only 2 scenarios for the cars : Either least congested lanes or the other lanes.

There is still a possibility that least congested cars can still reduce the transportation time by more than 25 mins and the transportation times of the cars in the other lanes less than 25mins and compensate for it - thereby causing the average to still be 25% more than trains/subways

[tim]

you're right. mathematically, we could have a public transportation 75 minute commute along with a 100 minute average for cars. maybe half the cars use the carpool lane and average 80 minutes but the other half use regular lanes and average 120 minutes. this is more than 25% time savings, so yes we can manipulate this one outside the constraints if we really try. keep in mind though that this is not a math problem, and instead we are only asked which conclusion the statements will "best support"..

[V_zenkova]

"...does not reduce driving time by more than 25 %" - Doesn't it mean that it still might reduce it by 25% (but not more)? In this case the answer is wrong as the time spent on carpool lane can be equal to time spent in public transport.

[jlucero]

"...does not reduce driving time by more than 25 %" - Doesn't it mean that it still might reduce it by 25% (but not more)? In this case the answer is wrong as the time spent on carpool lane can be equal to time spent in public transport.

That's a good thing in this answer choice. If you're trying to draw a conclusion, a non-extreme answer would help your cause. If another answer choice said driving in the carpool lane would not reduce your commute by 50%, that answer would be correct b/c we're saying it WON'T have an extreme affect. Carpooling could reduce your time by 10%, 20%, maybe even 24.9%, but definitely not more than 25%.