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# A driver completed the first 20 miles of a 40-mile trip at an average

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Intern
Joined: 11 Dec 2016
Posts: 47
Re: A driver completed the first 20 miles of a 40-mile trip at an average  [#permalink]

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12 Apr 2018, 10:41
1
VeritasPrepKarishma

I figured out. Sorry!

It's like:

t1/t2=(x-60)/(60-50)

Where t1= 2/5 & t2= 20/x

The equation looks something like this after solving a bit
2x/5*20=(x-60)/10
x=5(x-60)
X=5x-300
4x=300
x= 75

Option D
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Re: A driver completed the first 20 miles of a 40-mile trip at an average  [#permalink]

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12 Apr 2018, 20:18
1
1
asfandabid wrote:
VeritasPrepKarishma

I figured out. Sorry!

It's like:

t1/t2=(x-60)/(60-50)

Where t1= 2/5 & t2= 20/x

The equation looks something like this after solving a bit
2x/5*20=(x-60)/10
x=5(x-60)
X=5x-300
4x=300
x= 75

Option D

Though you have applied the scale method correctly, I would like to point out that the best method to use here is the formula 2ab/(a+b)
When one drives at speed a for half the distance and at speed b for the other half of the distance, the average speed is given by 2ab/(a+b)

60 = 2*50*b/(50 + b)
b = 3000/40 = 75

Check this post for these formulas: https://www.veritasprep.com/blog/2015/0 ... -the-gmat/
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Karishma
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Re: A driver completed the first 20 miles of a 40-mile trip at an average  [#permalink]

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02 Jul 2019, 07:45
b2bt wrote:
Average speed concept always confuses me. Anyways there is a formula

$$Avg. Speed = \frac{2(S_1)(S_2)}{S_1+S_2}$$

$$S_1$$ being one speed and $$S_2$$ being another

By substituting the value you will get 75 miles/hour

Time Taken - 1:45
Difficulty level - 600

COULD ANYONE PUT ALL CONCISE FORMULA TO SOLVE SUCH PROBLEM ? I UPLOAD BUT IT IS NOT COMPLETE !!!! IF YOU HAVE ALL FORMULA PLEASE POST IT .
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GMAT 1: 690 Q50 V34
WE: Engineering (Transportation)
Re: A driver completed the first 20 miles of a 40-mile trip at an average  [#permalink]

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02 Jul 2019, 08:26
Bunuel wrote:
A driver completed the first 20 miles of a 40-mile trip at an average speed of 50 miles per hour. At what average speed must the driver complete the remaining 20 miles to achieve an average speed of 60 miles per hour for the entire 40-mile trip? (Assume that the driver did not make any stops during the 40-mile trip.)

(A) 65 mph
(B) 68 mph
(C) 70 mph
(D) 75 mph
(E) 80 mph

Problem Solving
Question: 142
Category: Algebra Applied problems
Page: 80
Difficulty: 650

The Official Guide For GMAT® Quantitative Review, 2ND Edition

Let the speed required for second 20 miles be x mph.
Total time taken = 20/50 + 20/x = 20 (1/50 + 1/x) = 40/60 = 2/3
I/50 + 1/x = 1/30
x = 75 mph

IMO D
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A driver completed the first 20 miles of a 40-mile trip at an average  [#permalink]

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13 Jan 2020, 09:33
Hi karishma

Can you please explain whether the weighted average approach is possible to use in this question.

A similar question where the weighted average approach is used :https://gmatclub.com/forum/a-car-averages-40-miles-per-hour-for-the-first-6-hours-of-a-43216.html?fl=similar

Thanks
A driver completed the first 20 miles of a 40-mile trip at an average   [#permalink] 13 Jan 2020, 09:33

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