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# Susan drove an average speed of 30 miles per hour for the

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Susan drove an average speed of 30 miles per hour for the [#permalink]

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02 Nov 2010, 05:56
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Susan drove an average speed of 30 miles per hour for the first 30 miles of a trip & then at a average speed of 60 miles/hr for the remaining 30 miles of the trip if she made no stops during the trip what was susan's avg speed in miles/hr for the entire trip

A. 35
B. 40
C. 45
D. 50
E. 55
[Reveal] Spoiler: OA

Last edited by Bunuel on 13 Oct 2013, 02:01, edited 1 time in total.
Moved to PS forum.
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Re: susan drove an average speed [#permalink]

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02 Nov 2010, 06:04
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anilnandyala wrote:
susan drove an average speed of 30 miles per hour for the first 30 miles of a trip & then at a average speed of 60 miles/hr for the remaining 30 miles of the trip if she made no stops during the trip what was susan's avg speed in miles/hr for the entire trip

a 35
b 40
c 45
d 50
e 55

$$average \ speed=\frac{total \ distance \ covered}{total \ time}$$;

Total distance covered = 30 + 30 = 60 miles;
Total time = 30/30 + 30/60 = 3/2 hours;

$$average \ speed=\frac{total \ distance \ covered}{total \ time}=\frac{60}{\frac{3}{2}}=40$$.

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Re: Susan drove an average speed of 30 mph for the first 30 min [#permalink]

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19 Mar 2013, 21:05
Your reasoning is correct. Probably the answer is incorrect unless you have not changed either the question or the answer.
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Re: Susan drove an average speed of 30 mph for the first 30 min [#permalink]

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19 Mar 2013, 21:51
Avg. speed = total distance / total time

Total distance = 60 miles
Total time = 30 / 30 + 30/60 = 3/2

Avg. speed = 40.

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Re: Susan drove an average speed of 30 mph for the first 30 min [#permalink]

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20 Mar 2013, 01:09
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jsphcal wrote:
Susan drove an average speed of 30 miles per hour for the first 30 miles of a trip and then at an average speed of 60 miles per hour for the remaining 30 miles of the trip. If she made no stops during the trip, what was Susan's average speed, in miles per hour, for the entire trip?

a. 35
b. 40
c. 45
d. 50
e. 55

The Common Formula is Average speed = Total distance / Total time.
However if the distances traveled with two different speeds are the same (as 30 in this case), we can calculate the average speed as per following rule.
ratio of two distinct speeds = 1:2
Difference of two distinct speeds = 60 - 30 = 30
Now divide this difference by the sum of intergers of ratio (i.e. by 1+2 = 3 in this case ) 30/3 = 10
Average speed will be 10 X 1 part away from the lower speed i.e. 30 + 10 X 1 = 40.

The benefit of this method is once practiced enough you can calculate the average speed mentally or with minimal calculation and thereby can save valuable time on the GMAT

Regards,

Abhijit
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Last edited by Narenn on 20 Mar 2013, 03:56, edited 1 time in total.
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Re: Susan drove an average speed of 30 miles per hour for the [#permalink]

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25 Mar 2013, 10:40
My approach:

30/30 -> 1h
30/60 -> 0.5h
---------1.5h total and then 60/1.5 -> 40 (1.5 -> 3 -> 4.5 -> 6)
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Re: Susan drove an average speed of 30 miles per hour for the [#permalink]

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30 Jul 2013, 15:45
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Susan drove an average speed of 30 miles per hour for the first 30 miles of a trip & then at a average speed of 60 miles/hr for the remaining 30 miles of the trip if she made no stops during the trip what was susan's avg speed in miles/hr for the entire trip

What we have here are equal distances for both segments.

First segment: 30 miles/hour and covered 30 miles, therefore it took one hour.
Second segment: 60 miles/hour and covered 30 miles, therefore it took 1/2 hour.

(Total distance / total time)
(60 / [1hr+ 1/2hr])
(60 / 1.5) = 40 miles avg. speed.

A. 35
B. 40
C. 45
D. 50
E. 55

(B)

When don't we simply add the distances/speeds together to get the average?
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Re: Susan drove an average speed of 30 miles per hour for the [#permalink]

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13 Oct 2013, 00:54
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anilnandyala wrote:
Susan drove an average speed of 30 miles per hour for the first 30 miles of a trip & then at a average speed of 60 miles/hr for the remaining 30 miles of the trip if she made no stops during the trip what was susan's avg speed in miles/hr for the entire trip

A. 35
B. 40
C. 45
D. 50
E. 55

@Bunuel - This is not a DS Question, kindly tag it to PS section. Thanks.
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Re: Susan drove an average speed of 30 miles per hour for the [#permalink]

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13 Oct 2013, 02:01
saurabhprashar wrote:
anilnandyala wrote:
Susan drove an average speed of 30 miles per hour for the first 30 miles of a trip & then at a average speed of 60 miles/hr for the remaining 30 miles of the trip if she made no stops during the trip what was susan's avg speed in miles/hr for the entire trip

A. 35
B. 40
C. 45
D. 50
E. 55

@Bunuel - This is not a DS Question, kindly tag it to PS section. Thanks.

Moved to PS forum. Thank you.
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Re: Susan drove an average speed of 30 miles per hour for the [#permalink]

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08 Nov 2014, 23:42
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Re: Susan drove an average speed of 30 miles per hour for the [#permalink]

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03 Feb 2015, 15:58
Rate x Time = Distance
Going: 30 x 1 = 30
Returning: 30 x .5 = 30

Avg speed = Total distance/Total Time
=(30+30)/ (1+.5)
=40
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Re: Susan drove an average speed of 30 miles per hour for the [#permalink]

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10 Feb 2016, 20:17
Narenn wrote:
jsphcal wrote:
Susan drove an average speed of 30 miles per hour for the first 30 miles of a trip and then at an average speed of 60 miles per hour for the remaining 30 miles of the trip. If she made no stops during the trip, what was Susan's average speed, in miles per hour, for the entire trip?

a. 35
b. 40
c. 45
d. 50
e. 55

The Common Formula is Average speed = Total distance / Total time.
However if the distances traveled with two different speeds are the same (as 30 in this case), we can calculate the average speed as per following rule.
ratio of two distinct speeds = 1:2
Difference of two distinct speeds = 60 - 30 = 30
Now divide this difference by the sum of intergers of ratio (i.e. by 1+2 = 3 in this case ) 30/3 = 10
Average speed will be 10 X 1 part away from the lower speed i.e. 30 + 10 X 1 = 40.

The benefit of this method is once practiced enough you can calculate the average speed mentally or with minimal calculation and thereby can save valuable time on the GMAT

Regards,

Abhijit

Nice method, but why should it be 10 away from the lower speed and not from the higher speed?
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Re: Susan drove an average speed of 30 miles per hour for the [#permalink]

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20 May 2016, 01:47
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Why can't we get her average speed by weighting her speed by the proportion of the total distance realized at a certain speed?

e.g. why is 30(30/60)+60(30/60)=15+30 incorrect?

Thank you please let me know if i haven't been clear enough
Re: Susan drove an average speed of 30 miles per hour for the   [#permalink] 20 May 2016, 01:47
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# Susan drove an average speed of 30 miles per hour for the

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