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# Skier Lindsey Vonn completes a straight 300-meter downhill

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Manager
Status: mba here i come!
Joined: 07 Aug 2011
Posts: 206
Skier Lindsey Vonn completes a straight 300-meter downhill  [#permalink]

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26 Aug 2011, 12:26
3
11
00:00

Difficulty:

75% (hard)

Question Stats:

68% (02:56) correct 32% (03:26) wrong based on 230 sessions

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Skier Lindsey Vonn completes a straight 300-meter downhill run in t seconds and at an average speed of (x + 10) meters per second. She then rides a chairlift back up the mountain the same distance at an average speed of (x - 8) meters per second. If the ride up the mountain took 135 seconds longer than her run down the mountain, what was her average speed, in meters per second, during her downhill run?

(A) 10
(B) 15
(C) 20
(D) 25
(E) 30

Spoiler: :: Solution
Time taken for the ride down = 300/(x+10)
Time taken for the ride up = 300/(x-8)

Now 300/(x+10) +135 = 300/(x-8)
20/(x+10) +9 = 20/(x-8)

x=10 clearly satisfies this

speed for the downhill journey = x+10 = 20 m/sec

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Joined: 06 Sep 2013
Posts: 1705
Concentration: Finance

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31 Dec 2013, 10:20
5
MBAhereIcome wrote:
Skier Lindsey Vonn completes a straight 300-meter downhill run in t seconds and at an average speed of (x + 10) meters per second. She then rides a chairlift back up the mountain the same distance at an average speed of (x - 8) meters per second. If the ride up the mountain took 135 seconds longer than her run down the mountain, what was her average speed, in meters per second, during her downhill run?

(A) 10
(B) 15
(C) 20
(D) 25
(E) 30

Spoiler: :: Solution
Time taken for the ride down = 300/(x+10)
Time taken for the ride up = 300/(x-8)

Now 300/(x+10) +135 = 300/(x-8)
20/(x+10) +9 = 20/(x-8)

x=10 clearly satisfies this

speed for the downhill journey = x+10 = 20 m/sec

Backsolving is the preferred approach for this one

300/(x-8) - 300/(x+10) = 135

We are asked for the value of x+10

So x = 10
300/2 - 300 / 20 = 135

It satisfies

Hope it helps!
Cheers!
J
##### General Discussion
Director
Joined: 01 Feb 2011
Posts: 658

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26 Aug 2011, 21:35
1
x+10?

300/(x+10) + 135 = 300/(x-8)

solving this we get x^2+2x-120 =0

=> x = 10

=> x+10 = 20.

Intern
Joined: 22 Jul 2014
Posts: 15
Concentration: Social Entrepreneurship, Sustainability
Re: Skier Lindsey Vonn completes a straight 300-meter downhill  [#permalink]

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30 Jul 2015, 18:31
jlgdr wrote:
MBAhereIcome wrote:
Skier Lindsey Vonn completes a straight 300-meter downhill run in t seconds and at an average speed of (x + 10) meters per second. She then rides a chairlift back up the mountain the same distance at an average speed of (x - 8) meters per second. If the ride up the mountain took 135 seconds longer than her run down the mountain, what was her average speed, in meters per second, during her downhill run?

(A) 10
(B) 15
(C) 20
(D) 25
(E) 30

Spoiler: :: Solution
Time taken for the ride down = 300/(x+10)
Time taken for the ride up = 300/(x-8)

Now 300/(x+10) +135 = 300/(x-8)
20/(x+10) +9 = 20/(x-8)

x=10 clearly satisfies this

speed for the downhill journey = x+10 = 20 m/sec

Backsolving is the preferred approach for this one

300/(x-8) - 300/(x+10) = 135

We are asked for the value of x+10

So x = 10
300/2 - 300 / 20 = 135

It satisfies

Hope it helps!
Cheers!
J

This is way more simpler than the solution offered by MGMAT! Thanks
VP
Joined: 09 Mar 2016
Posts: 1287
Skier Lindsey Vonn completes a straight 300-meter downhill  [#permalink]

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01 Dec 2018, 04:49
Spidy001 wrote:
x+10?

300/(x+10) + 135 = 300/(x-8)

solving this we get x^2+2x-120 =0

=> x = 10

=> x+10 = 20.

can anyone explain step by step how from this 300/(x+10) + 135 = 300/(x-8) we got this x^2+2x-120 =0
Senior PS Moderator
Status: It always seems impossible until it's done.
Joined: 16 Sep 2016
Posts: 680
Re: Skier Lindsey Vonn completes a straight 300-meter downhill  [#permalink]

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03 Dec 2018, 00:33
1
dave13, this is a great example of patience ( Delaying the simplification by opening brackets)

$$\frac{300}{t} = (x + 10)$$ ... this is the distance time speed relation from her run downhill.

$$\frac{300}{(t+135)} = (x - 8)$$ ... this second relation comes from her chairlift uphill.

We need to solve for x and hence from the above two equations we need to eliminate t.

Let's substitute the value of $$t = \frac{300}{(x+10)}$$ in the second equation. Yes, it looks scary but things will get better now

$$\frac{300*(x + 10)}{(300+(x+10)*135)} = x - 8$$
$$300*(x+10) = 300*(x-8) + 135*(x+10)*(x-8)$$ Let's not start opening brackets yet as things will reduce.
$$60*(x+10) = 60*(x-8) + 27*(x+10)*(x-8)$$ 5 is common in all numbers hence it has been cancelled out
$$20*(x+10) = 20*(x-8) + 9*(x+10)*(x-8)$$ Now 3 is common'

Now we are ready to open brackets and simplify

$$20x + 200 = 20x - 160 + 9(x+10)(x-8)$$
$$360 = 9*(x+10)(x-8)$$ 9 can still cancel from both sides
$$40 = x^2 + 2x - 80$$ Finally opening the brackets
$$x^2 + 2x - 120 = 0$$

$$x^2 + 12x - 10x - 120 = 0$$
$$x(x+12)-10(x+12) = 0$$
$$(x-10)*(x+12) = 0$$

Hence x = 10 discarding the negative root

Hence the speed downhill was ( x + 10) or 20 /s

Key takeaway here is : smart calculation saves a lot of time

Best,

dave13 wrote:
can anyone explain step by step how from this 300/(x+10) + 135 = 300/(x-8) we got this x^2+2x-120 =0

_________________

Regards,

“Do. Or do not. There is no try.” - Yoda (The Empire Strikes Back)

Manager
Joined: 29 May 2017
Posts: 130
Location: Pakistan
Concentration: Social Entrepreneurship, Sustainability
Re: Skier Lindsey Vonn completes a straight 300-meter downhill  [#permalink]

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06 Dec 2018, 01:40
1
dave13 wrote:
Spidy001 wrote:
x+10?

300/(x+10) + 135 = 300/(x-8)

solving this we get x^2+2x-120 =0

=> x = 10

=> x+10 = 20.

can anyone explain step by step how from this 300/(x+10) + 135 = 300/(x-8) we got this x^2+2x-120 =0

I also got stuck with the algebra....its not difficult but it is LONG.
my takeaway: if you are sure of the basic setup of the equation AND it looks that it will get ugly, then it is faster and CLEANER to plug the answer choices than to solve for roots.

this is something i have to remind myself each time i start a question....to get into the habit.
Re: Skier Lindsey Vonn completes a straight 300-meter downhill &nbs [#permalink] 06 Dec 2018, 01:40
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