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# Susan travels by car at an average speed of 50 miles per

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GRE Prep Club Legend
Joined: 07 Jun 2014
Posts: 4809
GRE 1: Q167 V156
WE: Business Development (Energy and Utilities)
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Susan travels by car at an average speed of 50 miles per [#permalink]  24 Nov 2017, 20:39
Expert's post
00:00

Question Stats:

100% (00:28) correct 0% (00:00) wrong based on 18 sessions
Susan travels by car at an average speed of 50 miles per hour for 4 hours and then at an average speed of 20 miles per hour for 2 hours. What is her average speed, in miles per hour, for the entire 6-hour trip?

A. 25
B. 30
C. 35
D. 40
E. 45

Drill 1
Question: 4
Page: 521
[Reveal] Spoiler: OA

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Sandy
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Director
Joined: 20 Apr 2016
Posts: 961
WE: Engineering (Energy and Utilities)
Followers: 13

Kudos [?]: 747 [0], given: 158

Re: Susan travels by car at an average speed of 50 miles per [#permalink]  02 Dec 2017, 10:55
sandy wrote:
Susan travels by car at an average speed of 50 miles per hour for 4 hours and then at an average speed of 20 miles per hour for 2 hours. What is her average speed, in miles per hour, for the entire 6-hour trip?

A. 25
B. 30
C. 35
D. 40
E. 45

Drill 1
Question: 4
Page: 521

Here in the first part,

Distance = speed * Time

Distance = 50 * 4 = 200 miles (Speed = 50 miles per hour ; Time = 4 hrs)

in the second part

Distance = 20 * 2 = 40 miles (Speed = 20 miles per hour ; Time = 2 hrs)

Therefore Total distance = 240 miles.

Av speed for the entire trip = 240/6 = 40 miles per hour (Since total time = 6 hrs)
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GRE Prep Club Legend
Joined: 07 Jun 2014
Posts: 4809
GRE 1: Q167 V156
WE: Business Development (Energy and Utilities)
Followers: 128

Kudos [?]: 2045 [0], given: 397

Re: Susan travels by car at an average speed of 50 miles per [#permalink]  09 Dec 2017, 19:45
Expert's post
Explanation

Use the given averages to figure out Susan’s total distance: 4 hours at an average speed of 50 miles per hour is a total of 200 miles, and 2 hours at an average speed of 20 miles per hour is a total of 40 miles.

Susan goes a total of 240 miles in 6 hours, thus, her average speed is $$\frac{240}{6}$$ miles/hour, or 40 miles per hour.

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Sandy
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Intern
Joined: 24 Jan 2019
Posts: 26
Followers: 0

Kudos [?]: 2 [0], given: 29

Re: Susan travels by car at an average speed of 50 miles per [#permalink]  05 Feb 2019, 07:02
Why cant we use the avg speed equation in this problem? as when i tried it , it gave me a different answer.
Avg Speed = 2(S1.S2) / S1 + S2
And when is this equation applicable?

Thanks
Director
Joined: 20 Apr 2016
Posts: 961
WE: Engineering (Energy and Utilities)
Followers: 13

Kudos [?]: 747 [0], given: 158

Re: Susan travels by car at an average speed of 50 miles per [#permalink]  05 Feb 2019, 10:33
SKH121 wrote:
Why cant we use the avg speed equation in this problem? as when i tried it , it gave me a different answer.
Avg Speed = 2(S1.S2) / S1 + S2
And when is this equation applicable?

Thanks

That will be applicable when Distance travelled in first half = distance travelled in 2nd half
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Re: Susan travels by car at an average speed of 50 miles per   [#permalink] 05 Feb 2019, 10:33
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