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a car averages 40 mph for the first 6 hours of a trip and

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CEO
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a car averages 40 mph for the first 6 hours of a trip and  [#permalink]

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New post 16 Feb 2007, 03:18
a car averages 40 mph for the first 6 hours of a trip and averages 60mph each additional hour of travel time. if the average speed for the entire trip is 55mph, how many hours long is the trip?

answer: 24 hours



how do i set up this problem using the RTD method instead of the annoying POE method (which is why i didnt list the choices here..)?

on a side note, how to i search for exact phrases on this board? i'm sure this question has been posted since it is from the PR book.

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New post 16 Feb 2007, 03:39
Hi and welcome to GMATClub :D :) (even if I prefer Audi ;) )

Let set some variables
o d1 : done distance at 40 mph
o t1 : time spent at 40 mph

o d2 : done distance at 60 mph
o t2 : time spent at 60 mph

Then, we can put all in equations:
o t1 = 6
o d1/t1 = 40
o d2/t2 = 60
o (d1+d2)/(t1+t2) = 55

We have so:
o d1 = 40*6
o d2=60*t2

o d1 + d2 = 55*(t1+t2)
<=> 40*6 + 60*t2 = 55*6 + 55*t2
<=> 5*t2 = 15*6
<=> t2 = 3*6 = 18

Finally,
t1 + t2 = 18 + 6 = 24
Intern
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New post 16 Feb 2007, 04:12
1
What are RTD method and POE method?

Did the calculation like Fig

Here one img for clearer view
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CEO
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Posts: 2685
Location: New York City
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New post 16 Feb 2007, 04:28
thanks guys. :-D

i just passed CFA Level I and i am just getting back into study mode so my mind hasnt really adapted yet.

POE = process of elimination
RTD= rate x time = distance


i solved it using figs example and before i saw the 2nd response from the other poster. i simplified it into something i would understand.


Rate X Time = Distance
R x T = D

R1 x T1 = D1
40 x 6 = 240

R2 x T2 = D2
60 x X = D2



R = D / T
D = D1 + D2 = 240 + D2 = 240 + 60 X
T = 6 + X

55 = 240 + 60X / 6 + X

240 + 60X = 55 (6 + X)
X=18

18 + initial 6 hours = 24
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New post 16 Feb 2007, 05:05
Really, I don't think you should use POA here.

It's not difficult to calculate
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Re: a car averages 40 mph for the first 6 hours of a trip and  [#permalink]

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New post 09 Mar 2018, 14:26
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Re: a car averages 40 mph for the first 6 hours of a trip and &nbs [#permalink] 09 Mar 2018, 14:26
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