"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?

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!

_________________
Ben Ku
Instructor
ManhattanGMAT