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21 Jan 2019, 00:06
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A
Be sure to select an answer first to save it in the Error Log before revealing the correct answer (OA)!
Difficulty:
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Question Stats:
91% (01:14) correct
9%
(01:37)
wrong
based on 92
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Sarah goes on a 600-mile trip. She travels at a constant rate of 50 miles per hour for the first 300 miles of the trip, and at a constant rate of 100 miles per hour for the last 300 miles of the trip. What is Sarah’s average speed in miles per hour for the entire trip?
A 200/3
B 70
C 75
D 80
E 85
A 200/3
B 70
C 75
D 80
E 85
Chethan92
Joined: 18 Jul 2018
Last visit: 21 Apr 2022
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Location: India
Concentration: Operations, General Management
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GMAT 1: 590 Q46 V25
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21 Jan 2019, 00:12
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Average speed = Total distance/time taken
Average speed = 600/(300/50+300/100) = 600/9 = 200/3
A is the answer
Posted from my mobile device
Average speed = 600/(300/50+300/100) = 600/9 = 200/3
A is the answer
Posted from my mobile device
23 Jan 2019, 13:57
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Bunuel
(1) The trap is Answer C
Common trap and incorrect option C is the average of the averages: \(\frac{50+100}{2}=75\)
Rule: Even if distance is equal, we cannot average the average speeds.
(If TIME is equal, often we can average the average speeds.* That scenario will be rare.)
(2) Average speed is weighted
We need time traveled for each leg because
Average speed = \(\frac{TotalDistance}{TotalTime}\)
Leg 1, time?
\(r*t=D\)
\(50*t_1=300\)
\(t_1=6\) hours
Leg 2, time?
\(100*t_2=300\)
\(t_2=3\) hours
Total \(D = 600\)
Total \(t = t_1 + t_2 = 9\) hours
Average speed =
\(\frac{TotalDistance}{TotalTime}=\frac{600mi}{9hrs}=\frac{200}{3}\) mph
Answer A
*Example in which we can average the average speeds because time traveled is equal:
Jake drives at 40 mph for 2 hours, and at 60 mph for 2 hours. Average speed?
Leg 1: 40 mph * 2 hrs = 80 miles
Leg 2: 60 mph * 2 hrs = 120 miles
Total D = 200. Total time = 4
Average speed = Total D/total t= 200/4 = 50 mph = (40 + 60)/2
