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GMAT Prep Test: Susan Drove an Average Speed...

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Guest
 Post subject: GMAT Prep Test: Susan Drove an Average Speed...
 Post Posted: Tue Nov 04, 2008 4:13 pm 
Susan drove an average speed of 30 mph for the first 30 minutes of a trip. She did drove 60 mph for the next 30 minutes. If she made no stops, what was the average speed, in miles per hour, for the entire trip?

a. 35
b. 40
c. 45
d. 50
e. 55

I get answer C. The correct answer is listed as B.

30mph x 0.5 hours = 15 miles; 60mph x 0.5 hours = 30 miles
Total distance = 45 miles
Total time = 1 hour

Therefore average speed for the entire trip = 45 mph.

Can't figure out how to get 40.
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RonPurewal
 Post subject:
 Post Posted: Thu Nov 20, 2008 7:28 am 
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ManhattanGMAT Staff


Posts: 7146
your workup is entirely correct, leading me to believe that perhaps you misread the problem.

are you sure it didn't say this (changes in bold):
"Susan drove an average speed of 30 mph for the first 30 miles of a trip. She did drove 60 mph for the next 30 miles"
?

in this case:
time for first leg of trip = 30 miles / (30 mi/hr) = 1 hr
time for second leg of trip = 30 miles / (60 mi/hr) = 0.5 hr
total time = 1.5 hr

average speed = 60 mi / 1.5 hr = 40 mi/hr

if the problem really says what you've posted, though, then you're correct.
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Guest
 Post subject:
 Post Posted: Sun Nov 30, 2008 2:13 am 
It's definitely "MILES" :)
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