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David travels one-third of the distance from LA to SF at a s

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David travels one-third of the distance from LA to SF at a s [#permalink]

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New post Updated on: 08 Apr 2014, 08:58
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David travels one-third of the distance from LA to SF at a speed of 40 miles an hour and the balance at a speed of 80 miles an hour. What is the average speed of David's travel from LA to SF?

A. 66.67 miles per hour

B. 48 miles per hour

C. 64 miles per hour

D. 60 miles per hour

E. 53.33 miles per hour

Originally posted by Safal on 08 Apr 2014, 08:24.
Last edited by Bunuel on 08 Apr 2014, 08:58, edited 1 time in total.
Renamed the topic and edited the tags.
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Re: David travels one-third of the distance from LA to SF at a s [#permalink]

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New post 08 Apr 2014, 09:06
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Safal wrote:
David travels one-third of the distance from LA to SF at a speed of 40 miles an hour and the balance at a speed of 80 miles an hour. What is the average speed of David's travel from LA to SF?

A. 66.67 miles per hour

B. 48 miles per hour

C. 64 miles per hour

D. 60 miles per hour

E. 53.33 miles per hour


(Average speed) = (total distance)/(total time).

Say the distance from LA to SF is d miles, then (Average speed) = (total distance)/(total time) = \(\frac{d}{\frac{(\frac{d}{3})}{40}+\frac{(\frac{2d}{3})}{80}}=\frac{d}{\frac{d}{120}+\frac{d}{120}}=60\) miles per hour.

Answer: D.


P.S. Please name the topics properly. Check rule 3 here: rules-for-posting-please-read-this-before-posting-133935.html

Also, please tag the topics properly: this is a distance/rate problem, not arithmetic.

Thank you.


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Re: David travels one-third of the distance from LA to SF at a s [#permalink]

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New post 09 Apr 2014, 01:37
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Lets say he travels total 3 Miles........ (1)

So, speed for 1 mile = 40
Speed for next 2 miles = 80

Total time taken \(=\frac{1}{40} + \frac{2}{80} = \frac{2}{40}\)........ (2)

\(Average = \frac{Total distance}{Total time}\)

\(= 3 * \frac{40}{2}\)

= 60

Answer = D
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Re: David travels one-third of the distance from LA to SF at a s [#permalink]

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New post 15 Jun 2015, 08:07
Safal wrote:
David travels one-third of the distance from LA to SF at a speed of 40 miles an hour and the balance at a speed of 80 miles an hour. What is the average speed of David's travel from LA to SF?

A. 66.67 miles per hour

B. 48 miles per hour

C. 64 miles per hour

D. 60 miles per hour

E. 53.33 miles per hour


Another way to solve the question is by assuming the distance which will avoid the unwanted calculation of fractions

Let's assume the distance which is divisible by 3 as 1/3 Distance is Travelled @40 mph
also Let's assume a distance that is divisible by 40 and 80 as well as TIME = DISTANCE/SPEED and Speeds are 40 and 80 mph

Let, Distance = 240 Miles

1/3 Distance = (1/3)*240 = 80 miles is Travelled @40 mph

i.e. TIME TAKEN = Distance / Speed = 80/40 = 2 hours

2/3 Distance = (2/3)*240 = 160 miles is Travelled @80 mph

i.e. TIME TAKEN = Distance / Speed = 160/80 = 2 hours

TOTAL DISTANCE = 240
TOTAL TIME = 2+2 = 4 Hours

AVERAGE SPEED = TOTAL DISTANCE / TOTAL TIME = 240/4 = 60 mph

Answer: option
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Re: David travels one-third of the distance from LA to SF at a s [#permalink]

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New post 26 Aug 2017, 16:06
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Re: David travels one-third of the distance from LA to SF at a s   [#permalink] 26 Aug 2017, 16:06
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