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

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Joined: 23 Sep 2016
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27 Feb 2018, 22:38
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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

IMO B .i solved this question by box method attached bellow:-
KUDOS iF YOU LIKE MY EXPLANATION. THANKS IN ADVANCE
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03 Apr 2018, 10:19
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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 = (TOTAL distance)/(TOTAL time)

TOTAL distance
Susan traveled 30 miles at a speed of 30mph, and 30 miles more at a speed of 60mph.
So, TOTAL distance = 30 miles + 30 miles = 60 miles

TOTAL time
TOTAL time = (time spent driving 30 mph) + (time spent driving 60 mph)
time = distance/speed
- Time spent driving 30 mph = 30 miles/30mph = 1 hour
- Time spent driving 60 mph = 30 miles/60mph = 0.5 hours
TOTAL time = (1 hour) + (0.5 hours) = 1.5 hours
----------------------------------

So, Average speed = (TOTAL distance)/(TOTAL time)
= 60 miles/1.5 hours
= 40 mph

Cheers,
Brent
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Re: Susan drove an average speed of 30 miles per hour for the  [#permalink]

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14 Apr 2018, 23:04
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

Distance 1 = 30 miles

Speed 1 = 30 m/h

Time 1 = 30/30 = 1 hr

Distance 2 = 30 miles

Speed 2 = 60 m/h

Time 2 = 30/60 = 0.5 hr

Average speed = $$\frac{30 * 1 + 60 * 0.5}{1 + 0.5}$$

Avg Speed = 40 m/h

(B)
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Re: Susan drove an average speed of 30 miles per hour for the  [#permalink]

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01 Oct 2018, 10:52
If one is able to understand concepts of speed correctly. They can note that this is not the arithmetic mean ever(unless distance and speed for both journey's are equal). Averaging of speed works on Harmonic Mean. That is 2 x A x B) /(A+B)

Where A and B are 2 speed. Works always whether a person is following to-fro motion or running same distance with diff speeds.

Do Note : This doesn't work when 2 distances are different.
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13 Oct 2018, 06:45
Avg. Speed= 2ab/(a+b)= 2*30*60/(30+60) = 40 mph.

Ans-B
Re: Susan drove an average speed of 30 miles per hour for the   [#permalink] 13 Oct 2018, 06:45

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