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## A car travels from city a to city B, a distance of 240kms...

tagged by: swerve

This topic has 2 expert replies and 0 member replies

### Top Member

swerve Master | Next Rank: 500 Posts
Joined
29 Oct 2017
Posted:
229 messages

#### A car travels from city a to city B, a distance of 240kms...

Sun Dec 03, 2017 12:59 pm
A car travels from city a to city B, a distance of 240kms. The car travels half the distance at x kmph and the remaining distance at x+20 kmph. If the car has to complete the journey in less than 5 hours what should be the minimum value of x?

A) 20
B) 30
C) 40
D) 50
E) 60

The OA is C.

Please, can any expert explain this PS question for me? I have many difficulties to understand why that is the correct answer. Thanks.

### GMAT/MBA Expert

Scott@TargetTestPrep GMAT Instructor
Joined
25 Apr 2015
Posted:
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Fri Jan 12, 2018 6:38 am
swerve wrote:
A car travels from city a to city B, a distance of 240kms. The car travels half the distance at x kmph and the remaining distance at x+20 kmph. If the car has to complete the journey in less than 5 hours what should be the minimum value of x?

A) 20
B) 30
C) 40
D) 50
E) 60
We see that the first half of the journey takes 120/x hours and the second half takes 120/(x+20) hours. Assuming that the journey takes exactly 5 hours, we can create the following equation:

120/x + 120/(x+20) = 5

Multiplying the entire by x(x+20) we have:

12(x+20) + 120x = 5x(x+20)

120x + 2400 + 120x = 5x^2 + 100x

5x^2 - 140x - 2400 = 0

x^2 - 28x - 480 = 0

(x - 40)(x + 12) = 0

x = 40 or x = -12

Since x canâ€™t be negative, then x = 40, which must be the minimum value of x.

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Scott Woodbury-Stewart Founder and CEO

### GMAT/MBA Expert

Rich.C@EMPOWERgmat.com Elite Legendary Member
Joined
23 Jun 2013
Posted:
9195 messages
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GMAT Score:
800
Sun Dec 03, 2017 7:17 pm
Hi swerve,

We're told that a car travels a distance of 240 km. The car travels HALF the distance at X km/hr and the remaining distance at (X+20) km/hr. We're asked to find the MINIMUM value of X if the car has to complete the journey in less than 5 hours. This question can be solved by TESTing THE ANSWERS. Since the prompt asks for the MINIMUM value for X, we should start with Answer A. If that doesn't 'fit', we next Test Answer B, and so on. I'm going to do the first set of calculations, then you can finish the work to find the correct answer....
IF.... X = 20
The two speeds are 20 km/hr and 40 km/hr
120 km at 20 km/hr takes 6 hours
120 km at 40 km/hr takes 3 hours
Total time = 9 hours... but this is TOO BIG (it's supposed to be LESS than 5 hours), so Answer A is TOO SLOW.

What happens when you TEST Answer B? Does that 'fit' or do you have to continue working?

GMAT assassins aren't born, they're made,
Rich

_________________
Contact Rich at Rich.C@empowergmat.com

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