Distance problem

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kris610: Master | Next Rank: 500 Posts; Posts: 198; Joined: Fri Mar 09, 2007 8:40 pm; Thanked: 7 times

by kris610 » Thu Mar 17, 2011 8:22 pm

Robin drove from Townville to Villageton. Upon arriving in Villageton, she immediately returned to Townville. What was Robin's average speed for the entire trip?

(1) While driving from Townville to Villageton, Robin's average speed was 40 miles per hour. While driving from Villageton to Townville, Robin's average speed was 60 miles per hour.

(2) The distance from Townville to Villageton is 120 miles.

This is a pretty simple problem, but I noticed a possible error. Just wanna see if others pose the same question I had while solving this.

A

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Source: Beat The GMAT — Problem Solving | Thu Mar 17, 2011 8:22 pm

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Anurag@Gurome: GMAT Instructor; Posts: 3835; Joined: Fri Apr 02, 2010 10:00 pmFollowed by: 523 members; Location: Milpitas, CA; Thanked: 1854 times; GMAT Score:770

by Anurag@Gurome » Thu Mar 17, 2011 8:41 pm

kris610 wrote:Robin drove from Townville to Villageton. Upon arriving in Villageton, she immediately returned to Townville. What was Robin's average speed for the entire trip?

(1) While driving from Townville to Villageton, Robin's average speed was 40 miles per hour. While driving from Villageton to Townville, Robin's average speed was 60 miles per hour.

(2) The distance from Townville to Villageton is 120 miles.

This is a pretty simple problem, but I noticed a possible error. Just wanna see if others pose the same question I had while solving this.

A

Average speed = (total distance)/(total time).
Consider first (1) alone.
Let the distance from Townville to Villageton be d.
So total time taken is d/40 + d/60 .
So, average speed is (2d)/(d/40 + d/60) = 2*40*60/(40+60) = 48 miles per hour.
Or, (1) alone is sufficient.
Next consider (2) alone.
We know the distance but not time.
Or (2) alone is not sufficient.

The correct answer is hence (A).

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ankur.agrawal: Master | Next Rank: 500 Posts; Posts: 261; Joined: Wed Mar 31, 2010 8:37 pmFollowed by: 3 members; Location: Varanasi; Thanked: 11 times

by ankur.agrawal » Thu Mar 17, 2011 8:44 pm

Robin drove from Townville to Villageton. Upon arriving in Villageton, she immediately returned to Townville. What was Robin's average speed for the entire trip?

(1) While driving from Townville to Villageton, Robin's average speed was 40 miles per hour. While driving from Villageton to Townville, Robin's average speed was 60 miles per hour.

(2) The distance from Townville to Villageton is 120 miles.

1) T-V , Only speeds are given & average speed cannot be the simple average of two speeds.NS

2) Total Distance =120*2 =240. again NS.

Together: 120/ 40 = 3 hrs ; 120/60 = 2 hrs. Total time taken = 5 hrs.

Average Speed for the entire trip = 240/5 = 48 m/hr .

Possible flaw: Question doesn't states that robin returns from the same route . Wats the source?

I feel answer should be C

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kris610: Master | Next Rank: 500 Posts; Posts: 198; Joined: Fri Mar 09, 2007 8:40 pm; Thanked: 7 times

by kris610 » Thu Mar 17, 2011 8:52 pm

Thanks Ankur. The source is beatthegmat's practice questions.

Anurag, I had the same question a Ankur. Where does the question state/indicate that the return trip was along the same route? The equation d/40 + d/60 becomes flawed if the return trip was not along the same route.

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Anurag@Gurome: GMAT Instructor; Posts: 3835; Joined: Fri Apr 02, 2010 10:00 pmFollowed by: 523 members; Location: Milpitas, CA; Thanked: 1854 times; GMAT Score:770

by Anurag@Gurome » Thu Mar 17, 2011 9:06 pm

kris610 wrote:Thanks Ankur. The source is beatthegmat's practice questions.

Anurag, I had the same question a Ankur. Where does the question state/indicate that the return trip was along the same route? The equation d/40 + d/60 becomes flawed if the return trip was not along the same route.

It's a point to be noted.
I agree!
They should specify that the same route is taken.

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