Tara drove at an average speed of 50 miles per hour for the first 50 miles of her trip and then at an average speed of 75 miles per hour for the remaining 50 miles of her trip. If she made no stops during the trip, what was Tara's average speed, in miles per hour, for the entire trip?
(A) 50
(B) 55
(C) 60
(D) 65
(E) 70
The OA is C.
I don't have this PS question clear. How can I find the average speed?
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Tara drove at an average speed of 50 miles per hour
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GMATGuruNY
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Since the distance traveled at each speed is THE SAME, the distance can be ANY VALUE.Vincen wrote:Tara drove at an average speed of 50 miles per hour for the first 50 miles of her trip and then at an average speed of 75 miles per hour for the remaining 50 miles of her trip. If she made no stops during the trip, what was Tara's average speed, in miles per hour, for the entire trip?
(A) 50
(B) 55
(C) 60
(D) 65
(E) 70
To make the math easier, let the distance at each speed = the LCM of 50 and 75 = 150 miles.
At a speed of 50mph, the time to travel 150 miles = d/r = 150/50 = 3 hours.
At a speed of 75mph, the time to travel 150 miles = d/r = 150/75 = 2 hours.
Average speed for the whole trip = (total distance)/(total time) = (150+150)/(3+2) = 300/5 = 60mph.
The correct answer is C.
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Scott@TargetTestPrep
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We can use the following formula:Vincen wrote:Tara drove at an average speed of 50 miles per hour for the first 50 miles of her trip and then at an average speed of 75 miles per hour for the remaining 50 miles of her trip. If she made no stops during the trip, what was Tara's average speed, in miles per hour, for the entire trip?
(A) 50
(B) 55
(C) 60
(D) 65
(E) 70
The OA is C.
I don't have this PS question clear. How can I find the average speed?
average speed = (distance 1 + distance 2)/(time 1 + time 2)
We see that time 1 = 1 hour, time 2 = 50/75 = 2/3 hours, and distance 1 = distance 2 = 50 miles. Thus:
average = (50 + 50 )/(1 + 2/3)
average = 100/(5/3) = 300/5 = 60 mph.
Answer: C
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Mo2men
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Dear GMATGuru,GMATGuruNY wrote:Since the distance traveled at each speed is THE SAME, the distance can be ANY VALUE.Vincen wrote:Tara drove at an average speed of 50 miles per hour for the first 50 miles of her trip and then at an average speed of 75 miles per hour for the remaining 50 miles of her trip. If she made no stops during the trip, what was Tara's average speed, in miles per hour, for the entire trip?
(A) 50
(B) 55
(C) 60
(D) 65
(E) 70
To make the math easier, let the distance at each speed = the LCM of 50 and 75 = 150 miles.
At a speed of 50mph, the time to travel 150 miles = d/r = 150/50 = 3 hours.
At a speed of 75mph, the time to travel 150 miles = d/r = 150/75 = 2 hours.
Average speed for the whole trip = (total distance)/(total time) = (150+150)/(3+2) = 300/5 = 60mph.
The correct answer is C.
You have mentioned before that in some cases =, we can apply that an average speed is little that average arithmetic mean of the two speeds. i.e. average speed < (50+75)/2... then average speed < 62.5 So it is 60.
Is above correct? when can I apply this rule and when I CANNOT?
Thanks
