EnterMatrix wrote:
A car travels from Mayville to Rome at an average speed of 30 miles per hour and returns immediately along the same route at an average speed of 40 miles per hour. Of the following, which is closest to the average speed, in miles per hour, for the round trip?
A) 32.0
B) 33.0
C) 34.3
D) 35.5
E) 36.5
APPROACH #1: Apply the average speed formula
Average speed formula = (
total distance travelled)/(
total travel time)
If we let
d = the distance from Mayville to Rome, then
total distance travelled = 2d (since the car traveled from Mayville to Rome and then back)time = distance/rate So, the travel time from Mayville to Rome =
d/30Similarly, the travel time from Rome to Mayville =
d/40So, the
total travel time = d/30 + d/40 = 4d/120 + 3d/120 = 7d/120The average speed for the round trip= (
2d)/(
7d/120)
= (2d)(120/7d)
= 240d/7d
= 240/7 = 34 2/7 ≈ 34.3
Answer: C
APPROACH #2: Assign a "nice" distance between Mayville and Rome
So, let's say the distance between Mayville and Rome =
120 miles - since this distance that works well with 30 miles per hour AND 40 miles per hourTime = distance/speedSo, time from Mayville and Rome (at 30 mph) = 120/30 =
4 hours
Time from Rome to Mayville (at 40 mph) = 120/40 =
3 hours
Average speed = (total distance traveled)/(total time)
= (
120 +
120)/(
4 +
3)
= 240/7 = 34 2/7 ≈ 34.3
Answer: C
Cheers.
Brent
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Brent Hanneson – Creator of gmatprepnow.com
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