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 Post subject: GMAT Motion
PostPosted: Tue Aug 28, 2012 10:23 am 
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Joined: Tue Apr 13, 2010 8:48 am
Posts: 478
Jerry travels 8 miles at an average speed of 40 miles per hour, stops for 10 minutes, and then travels another 20 miles at an average speed of 60 miles per hour. What is Jerry’s average speed, in miles per hour, for this trip?

A. 40
B. 42.5
C. 44
D. 50
E. 52.5

(A) Since Rate = Distance/Time, the average rate at which Jerry travels over a certain distance can be found by dividing the total distance of the trip by the total time for the trip.

Let’s find the times for the various portions of the trip using the formula Time = Distance / Rate:
Traveling 8 miles at a speed of 40 miles per hour takes 8/40 = 1/5 hour.
Similarly, traveling 20 miles at 60 miles per hour takes 20/60 = 1/3 hour.
The 10 minutes he was stopped is equal to 1/6 of an hour.

Adding all these times together gives us the total time:
Total time = 1/5 + 1/3 + 1/6
= 6/30 +10/30 + 5/30 = 21/30
= 7/10 hour.

The total distance traveled is 28 miles.

Therefore, the average speed is:
= 28 / (7/10) = 28 × (10/7) = 40 miles per hour.

The correct answer is choice (A).
-----------
Hi,
I am not sure why in calculation of the total time you include 10 minutes that he stopped for? I thought the total time that we need is the time spent traveling (i.e moving)


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 Post subject: Re: GMAT Motion
PostPosted: Tue Aug 28, 2012 10:33 am 
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Joined: Fri Apr 09, 2010 2:11 pm
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Quote:
I am not sure why in calculation of the total time you include 10 minutes that he stopped for? I thought the total time that we need is the time spent traveling (i.e moving)
If we calculate the average speed while moving, then we will not include stopping time. But since we are calculating the average speed for the trip, we must use all time of the trip.


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