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18 Jun 2011, 04:32
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B
Be sure to select an answer first to save it in the Error Log before revealing the correct answer (OA)!
Difficulty:
45%
(medium)
Question Stats:
71% (01:57) correct
29%
(01:39)
wrong
based on 93
sessions
History
Date
Time
Result
Not Attempted Yet
A car travels from point A to point B. The average speed of the car is 60 miles/hr and it travels the first half of the trip at a speed of 90 mi/hr. What is the speed of the car in the second half of the trip?
A. 30
B. 45
C. 60
D. 75
E. 90
OPEN DISCUSSION OF THIS QUESTION IS HERE: a-car-travels-from-point-a-to-point-b-the-average-speed-of-98076.html
A. 30
B. 45
C. 60
D. 75
E. 90
OPEN DISCUSSION OF THIS QUESTION IS HERE: a-car-travels-from-point-a-to-point-b-the-average-speed-of-98076.html
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18 Jun 2011, 05:19
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Avg Speed = Total Distance / Total time.
Let total distance = d
Let the speed of the car in the second half of the trip = S
Time taken to travel first half (d/2) at speed of 90m/hr t1= d/2*90 = d/180
Time taken to travel second half (d/2) at speed of s m/hr t2= d/2*s = d/2s
Total time = t1+t2 = d/180 + d/2s
Avg Speed = Total Distance / Total time.
60 = d/(t1+t2)
\(60=d/[(d/180) + d/2s)]\)
60 = 360s/(2s+180)
s=45
OA B.
Let total distance = d
Let the speed of the car in the second half of the trip = S
Time taken to travel first half (d/2) at speed of 90m/hr t1= d/2*90 = d/180
Time taken to travel second half (d/2) at speed of s m/hr t2= d/2*s = d/2s
Total time = t1+t2 = d/180 + d/2s
Avg Speed = Total Distance / Total time.
60 = d/(t1+t2)
\(60=d/[(d/180) + d/2s)]\)
60 = 360s/(2s+180)
s=45
OA B.
18 Jun 2011, 06:18
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guygmat
By weighted average:
Let the speed be S;
\(\frac{\frac{1}{S}-\frac{1}{60}}{\frac{1}{60}-\frac{1}{90}}=\frac{x}{x}=1\)
Upon solving;
S=45 m/h
Ans: "B"
