An absent minded scientist walks on an escalator at a rate : GMAT Problem Solving (PS)
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# An absent minded scientist walks on an escalator at a rate

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An absent minded scientist walks on an escalator at a rate [#permalink]

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08 Nov 2010, 02:19
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An absent minded scientist walks on an escalator at a rate of 4 steps per second and reaches the other end in 15 seconds. While coming back, walking at the same speed he reaches the starting point in 45 seconds. What is the number of steps on the escalator?

A. 60
B. 90
C. 105
D. 120
E. 140
[Reveal] Spoiler: OA

Last edited by Bunuel on 09 Jul 2013, 08:02, edited 1 time in total.
Renamed the topic and edited the question.
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Re: an absent minded scientist [#permalink]

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08 Nov 2010, 02:40
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anilnandyala wrote:
an absent minded scientist walks on an escalator at a rate of 4 steps per second and reaches the other end in 15 seconds . while coming back , walking at the same speed he reaches the starting point in 45 seconds . what is the number of steps on the escalator?
a 60
b 90
c 105
d 120
e 140

Let the rate of the escalator be $$x$$ steps per second and the length of the escalator $$y$$ steps.

Then $$\frac{y}{x+4}=15$$ and $$\frac{y}{4-x}=45$$ --> solving for $$y$$ --> $$y=90$$.

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08 Nov 2010, 02:54
Scientist is absent minded. He uses same escalator while walking in both directions

Let S be speed of escalator

Case1-->same direction as that of escalator, so D/(4+S)= 15
Case2-->against direction of escalator, so D/(4-S) = 45
Combining above 2 equations- 15(4+S)= 45(4-S), S=2

Therefore number of stairs= 90. Answer: B.
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Re: an absent minded scientist [#permalink]

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08 Nov 2010, 07:09
Bunuel wrote:
anilnandyala wrote:
an absent minded scientist walks on an escalator at a rate of 4 steps per second and reaches the other end in 15 seconds . while coming back , walking at the same speed he reaches the starting point in 45 seconds . what is the number of steps on the escalator?
a 60
b 90
c 105
d 120
e 140

Let the rate of the escalator be $$x$$ steps per second and the length of the escalator $$y$$ steps.

Then $$\frac{y}{x+4}=15$$ and $$\frac{y}{4-x}=45$$ --> solving for $$y$$ --> $$y=90$$.

Is this a GMAT question.
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Re: an absent minded scientist [#permalink]

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08 Nov 2010, 08:38
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anilnandyala wrote:
an absent minded scientist walks on an escalator at a rate of 4 steps per second and reaches the other end in 15 seconds . while coming back , walking at the same speed he reaches the starting point in 45 seconds . what is the number of steps on the escalator?
a 60
b 90
c 105
d 120
e 140

Escalator questions tend to confuse people but the point to note here is that they are just like Boats and Streams questions. If you are going in the direction of the escalator, it is like going downstream and your effective speed will be your speed + escalator speed. When you are going against the escalator, it is like going upstream and your effective speed will be your speed - escalator speed. The total distance to be traveled is the total number of steps on the escalator.
Then, you can very easily make the equations made by Bunuel and Sarang above:
$$\frac{y}{x+4}=15 and \frac{y}{4-x}=45$$

You can take a couple of quick guesses to solve this and in most cases you will get your answer in a couple of guesses.
e.g. lets say if x = 1, y = 75 from 1st equation
but x = 1 gives y = 135 in 2nd equation. So x is not 1
If x = 2, y = 90, from 1st equation
x = 2 gives y = 90 in 2nd equation. So x = 2 and y = 90.
Fun fact about GMAT: They give you numbers that make your life easy but twist concepts to make you think.

Also, if I am not wrong, these are CAT questions. I have not seen such questions in OG/Test prep material that I have come across. Nevertheless, every question is good for practice if you have the time. If time is less, you would do better to focus on 'GMAT type' questions available in OG.
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Get started with Veritas Prep GMAT On Demand for $199 Veritas Prep Reviews Math Expert Joined: 02 Sep 2009 Posts: 36552 Followers: 7078 Kudos [?]: 93158 [0], given: 10553 Re: An absent minded scientist walks on an escalator at a rate [#permalink] ### Show Tags 09 Jul 2013, 08:05 Bumping for review and further discussion. _________________ Senior Manager Joined: 13 May 2013 Posts: 472 Followers: 3 Kudos [?]: 160 [0], given: 134 Re: An absent minded scientist walks on an escalator at a rate [#permalink] ### Show Tags 05 Aug 2013, 11:16 An absent minded scientist walks on an escalator at a rate of 4 steps per second and reaches the other end in 15 seconds. While coming back, walking at the same speed he reaches the starting point in 45 seconds. What is the number of steps on the escalator? It took me a while to figure out why the fact that the scientist was "absent minded" was relevant. He walked on the escalator the right way then went in the opposite direction against the direction it was moving in! We are given time and rate for both directions. We have to realize that because the escalator is moving, that adds (or subtracts) from his forward momentum. time = distance/speed t1: 15 = d/(r+4) 15r+60 = d t2: 45 = d/(4-r) The scientist moves at the same speed but because he is going against the motion of the escalator his forward movement is slowed down. Therefore, we subtract the escalators rate from his rate of 4 steps/second. 180-45r = d 15r+60 = 180-45r 60r - 120 r=2 15 = d/(r+4) 15 = d(2+4) 15 = d(6) 90 = d ANSWER: B. 90 Manager Joined: 14 Nov 2008 Posts: 70 Followers: 2 Kudos [?]: 23 [0], given: 1 Re: An absent minded scientist walks on an escalator at a rate [#permalink] ### Show Tags 01 Sep 2013, 02:34 T = total no. of steps in escalator n = speed of escalator in steps per second 4 = steps per sec of the scientist (n+4)=T/15 and (4-n) = T/45 => n = 2 => T = 90 Intern Joined: 26 Aug 2013 Posts: 1 Followers: 0 Kudos [?]: 0 [0], given: 0 Re: An absent minded scientist walks on an escalator at a rate [#permalink] ### Show Tags 02 Sep 2013, 06:24 As we can see, ratio of time take when going with the escalator to time taken against the escalator is 3 Also it is understandable that, speed of the escalator must be less then the speed of the scientist ( as the scientist is able to come down while moving against the escalator ) Futher is we assume X (steps/sec) be the avg. speed of the escalator then (4+X)/(4-X) = 3 i.e. X=2 Hence no of steps = (4+X)*15 = (4-X)*45 = 90 GMAT Club Legend Joined: 09 Sep 2013 Posts: 13444 Followers: 575 Kudos [?]: 163 [0], given: 0 Re: An absent minded scientist walks on an escalator at a rate [#permalink] ### Show Tags 31 Oct 2014, 03:07 Hello from the GMAT Club BumpBot! Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos). Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email. _________________ Director Joined: 07 Aug 2011 Posts: 581 Concentration: International Business, Technology GMAT 1: 630 Q49 V27 Followers: 3 Kudos [?]: 406 [0], given: 75 Re: An absent minded scientist walks on an escalator at a rate [#permalink] ### Show Tags 02 Mar 2015, 07:11 anilnandyala wrote: An absent minded scientist walks on an escalator at a rate of 4 steps per second and reaches the other end in 15 seconds. While coming back, walking at the same speed he reaches the starting point in 45 seconds. What is the number of steps on the escalator? A. 60 B. 90 C. 105 D. 120 E. 140 time taken going up 45sec time taken going down 15sec it tells going down speed is 3times the speed going up. 4+E=3(4-E) E=2 in 15secs scientist will move 60 steps in 15secs escalator will move 2×15=30 steps answer 90 _________________ Thanks, Lucky _______________________________________________________ Kindly press the to appreciate my post !! SVP Joined: 17 Jul 2014 Posts: 2187 Location: United States (IL) Concentration: Finance, Economics Schools: Stanford '19 (S) GMAT 1: 560 Q42 V26 GMAT 2: 550 Q39 V27 GMAT 3: 560 Q43 V24 GMAT 4: 650 Q49 V30 GPA: 3.92 WE: General Management (Transportation) Followers: 19 Kudos [?]: 270 [0], given: 140 Re: An absent minded scientist walks on an escalator at a rate [#permalink] ### Show Tags 20 Mar 2016, 17:15 to be honest..I did not even know how to approach this question... 4*15 = 60 steps 4*45 = 180 steps he makes 120 steps more in 30 additional seconds. if he skipped every other step while going up - he would have walked 90 steps if he walked 2 times backwards the same steps, then he walked 180. that's the only way i got to 90..but don't understand how it works in practice... Senior Manager Joined: 07 Dec 2014 Posts: 491 Followers: 3 Kudos [?]: 81 [0], given: 2 An absent minded scientist walks on an escalator at a rate [#permalink] ### Show Tags 24 Mar 2016, 12:22 let r=rate of escalator (4+r)(15)=(4-r)(45) r=2 steps per second (4+2)(15)=(4-2)(45)=90 steps on escalator Veritas Prep GMAT Instructor Joined: 16 Oct 2010 Posts: 7125 Location: Pune, India Followers: 2134 Kudos [?]: 13646 [0], given: 222 Re: An absent minded scientist walks on an escalator at a rate [#permalink] ### Show Tags 28 Mar 2016, 21:52 mvictor wrote: to be honest..I did not even know how to approach this question... 4*15 = 60 steps 4*45 = 180 steps he makes 120 steps more in 30 additional seconds. if he skipped every other step while going up - he would have walked 90 steps if he walked 2 times backwards the same steps, then he walked 180. that's the only way i got to 90..but don't understand how it works in practice... If you want to approach it this way, here is how to get it: Scientist takes 4*15 steps while going in the direction of the escalator. He is taking 60 steps and the escalator is taking some additional steps for him. He takes 4*45 = 180 steps while going against the direction of the escalator. He is taking 180 steps because the escalator is cancelling out some of his steps. The difference in these two numbers is 180 - 60 = 120. This 120 is the sum of the number of steps the escalator adds and cancels out. The escalator's speed is constant. It makes the same number of steps every second. So in 45 secs, it will make 3 times the number of steps that it does in 15 secs. So if it makes s steps in 15 secs (add s steps to the scientist), it will make 3s steps in 45 secs (will cancel out 3s steps of the scientist). s + 3s = 120 s = 30 So total steps of the escalator are 60 + 30 = 90 or total steps of the escalator = 180 - 3*30 = 90 _________________ Karishma Veritas Prep | GMAT Instructor My Blog Get started with Veritas Prep GMAT On Demand for$199

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Re: An absent minded scientist walks on an escalator at a rate   [#permalink] 28 Mar 2016, 21:52
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# An absent minded scientist walks on an escalator at a rate

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