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01 Oct 2018, 08:20
Kudos
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C
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Difficulty:
35%
(medium)
Question Stats:
72% (01:56) correct
28%
(01:17)
wrong
based on 416
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Jenny drove to the gym with an average speed of 60 miles per hour. On her way back she drove the same route with an average speed of 80 miles per hour. What was er average speed for the whole trip in mph?
a) 60
b) 68
c) 68 4/7
d) 70
e) 74 3/8
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a) 60
b) 68
c) 68 4/7
d) 70
e) 74 3/8
source gmat tutor
01 Oct 2018, 08:35
Kudos
Bookmarks
bettatantalo
Direct formula...
\(\frac{2*a*b}{a+b}=\frac{2*60*80}{60+80}\)=960/14=480/7=68 4/7
C
Otherwise..
Let the distance be d..
d/60 +d/80 =3d+4d/240.. = total time taken
Average speed = 2d/(7d/240)=480/7
C
01 Oct 2018, 10:23
Kudos
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bettatantalo
Another approach is to assign a "nice" value (one that works well with 60 mph and 80 mph) to the distance to the gym.
So, let's say the distance is 480 miles (a long commute indeed!)
Average speed = (total distance traveled)/(total travel time)
TOTAL distance = 480 miles + 480 miles = 960 miles
Now let's calculate the TOTAL travel time.
time = distance/speed
So, travel time TO the gym = 480/60 = 8 hours
And travel time FROM the gym = 480/80 = 6 hours
So, TOTAL travel time = 8 hours + 6 hours = 14 hours
So, average speed = 960 miles/14 hours
= 480/7 miles per hour
= 68 4/7 miles per hour
Answer: C
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