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GMAT PREP (Average speed & rate)

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GMAT PREP (Average speed & rate)

by relaxin99 » Mon Feb 16, 2009 4:39 pm
Susan drove at an average speed of 30 miles per hour for the first 30 miles of a trip and then at an average speed of 60 miles per hour for the remaining 30 miles of the trip. If she made no stops durign the trip, what was Susan's average speed, in miles per hour, for the entire trip?

A) 35
B) 40
C) 45
D) 50
E) 55

I get C, but that is wrong.........

Can someone please explain their answer as well, the correct answer is down below
























The answer is B 40
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by billzhao » Mon Feb 16, 2009 5:03 pm
To find the average speed V, we need to find the total distance:D she drived and total time: T she spent.

From the question, we know that D=30+30=60 miles
T=30/30+30/60=1.5 hours

So V=D/T=60/1.5=40 miles/hours

Answer is (B)
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by truplayer256 » Mon Feb 16, 2009 5:04 pm
Average speed= total distance/total time.
Total Distance that Susan covers is 60 miles.
Since she travels 30 mph for the first 30 miles, it takes 1 hour.
Since she travels 60 mph for the remaining 30 miles, it takes her half an hour.

(60)/(1+1/2)=(60)/(3/2)=40 mph
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