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If a car went the first third of the distance at 80 kmh, the

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If a car went the first third of the distance at 80 kmh, the  [#permalink]

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New post Updated on: 09 Jul 2013, 07:06
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If a car went the first third of the distance at 80 kmh, the second third at 24 kmh, and the last third at 48 kmh, what was the average speed of the car for the entire trip?

A. 36 kmh
B. 40 kmh
C. 42 kmh
D. 44 kmh
E. 50 kmh

(C) 2008 GMAT Club - m15#32

Originally posted by sudimba on 08 Sep 2010, 23:29.
Last edited by Bunuel on 09 Jul 2013, 07:06, edited 1 time in total.
Renamed the topic and edited the question.
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Re: gmat ps gmatclub iphone app 69/111  [#permalink]

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New post 08 Sep 2010, 23:34
1/80 + 1/24 + 1/48 = 18/240 = 3/40

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Re: gmat ps gmatclub iphone app 69/111  [#permalink]

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New post 08 Sep 2010, 23:35
sudimba wrote:
If a car went the first third of the distance at 80 kmh, the second third at 24kmh and the last third at 48kmh, what was the average speed of the car for the entire trip?

I know how to solve it but when I looked at the solution:

"Let S denote the whole distance. The time it took the car to finish the trip = (s/3)/80 + (s/3)/24 + (s/3)/48 = s/3 * 3/40

how's it possible to go from what's blue to what's in green?

Thanks!


(s/3)/80 + (s/3)/24 + (s/3)/48 --> factor out \(\frac{s}{3}\) --> \(\frac{s}{3}(\frac{1}{80}+\frac{1}{24}+\frac{1}{48})=\frac{s}{3}*\frac{3+10+5}{240}=\frac{s}{3}*\frac{18}{240}=\frac{s}{3}*\frac{3}{40}=\frac{s}{40}\).

Hope it helps.
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Re: gmat ps gmatclub iphone app 69/111  [#permalink]

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New post 11 Sep 2010, 21:37
i prefer starting with total distance of 3d...it makes calculation easier
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Re: gmat ps gmatclub iphone app 69/111  [#permalink]

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New post 13 Sep 2010, 03:52
So average speed is 40 isnt it???
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New post 13 Sep 2010, 08:29
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New post 21 Sep 2010, 18:08
yeah average speed = 40
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Re: If a car went the first third of the distance at 80 kmh, the  [#permalink]

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New post 06 Oct 2014, 14:24
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I don't know if I got lucky with smart numbers but....

\(Average Speed = \frac{Total Distance}{Total Time}\)

Set total distance: 30

That means each third had a distance of 10.

Calculate time for each third and sum them to get total time: \(\frac{10}{80} + \frac{10}{24} + \frac{10}{48} = \frac{3}{4}\)

Back to the original equation...

\(Average Speed = \frac{30}{3/4}\) which simplifies to \(\frac{30 * 4}{3}\) = 40
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Re: If a car went the first third of the distance at 80 kmh, the  [#permalink]

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New post 08 Oct 2014, 00:50
\(Average Speed = \frac{1}{\frac{\frac{1}{3}}{80} + \frac{\frac{1}{3}}{24} + \frac{\frac{1}{3}}{48}} = \frac{1}{\frac{1}{240} + \frac{1}{72} + \frac{1}{144}} = \frac{1}{\frac{3+10+5}{720}} = \frac{720}{18} = 40\)

Answer = B
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Re: If a car went the first third of the distance at 80 kmh, the  [#permalink]

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New post 22 Oct 2014, 10:40
sudimba wrote:
If a car went the first third of the distance at 80 kmh, the second third at 24 kmh, and the last third at 48 kmh, what was the average speed of the car for the entire trip?

A. 36 kmh
B. 40 kmh
C. 42 kmh
D. 44 kmh
E. 50 kmh

(C) 2008 GMAT Club - m15#32


Let total distance be 90km

90/ ( (3/8)+(5/4)+(5/8))= 40kmph
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If a car went the first third of the distance at 80 kmh, the  [#permalink]

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New post 24 Oct 2014, 02:45
Formula for Average speed for calculating 3 speeds= \(\frac{3ABC}{AB + BC + AC}\) (where A,B and C are 3 different speeds)

here- \(\frac{3 * 80 * 24 * 48}{(80 * 24) + (24 * 48) + (80 * 48)}\)


then,\(\frac{3 * 80 * 24 * 48}{24 (80 + 48 + 160 )}\)


then, \(\frac{3 * 80 * 48}{288}\)



then,\(\frac{3 * 80}{6}\) =\(40 km\)


P.S - When 2 speeds are given and average speed is to be calculated, use the following formula=

2AB / A + B
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If a car went the first third of the distance at 80 kmh, the  [#permalink]

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New post 27 May 2018, 21:31
sudimba wrote:
If a car went the first third of the distance at 80 kmh, the second third at 24 kmh, and the last third at 48 kmh, what was the average speed of the car for the entire trip?

A. 36 kmh
B. 40 kmh
C. 42 kmh
D. 44 kmh
E. 50 kmh


3/(1/80+1/24+1/48)=(3*240)/(3+10+5)=40 kmh average speed
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Re: If a car went the first third of the distance at 80 kmh, the  [#permalink]

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New post 11 Jul 2018, 23:26
PareshGmat wrote:
\(Average Speed = \frac{1}{\frac{\frac{1}{3}}{80} + \frac{\frac{1}{3}}{24} + \frac{\frac{1}{3}}{48}} = \frac{1}{\frac{1}{240} + \frac{1}{72} + \frac{1}{144}} = \frac{1}{\frac{3+10+5}{720}} = \frac{720}{18} = 40\)

Answer = B



What is the fastest way to solve this fraction? I find an LCD by prime factorising but it takes a long time. Any tips on the most effective method would be really appreciated.

THanks,
Mark
Re: If a car went the first third of the distance at 80 kmh, the &nbs [#permalink] 11 Jul 2018, 23:26
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