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Gears X and Y are interconnected within and power
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28 Feb 2011, 08:28
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64% (02:35) correct 37% (01:29) wrong based on 200 sessions
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Gears X and Y are interconnected within and power
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Re: Gears X and Y are interconnected within and power
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28 Feb 2011, 18:16
GMATD11 wrote: How to approach such type of question? Such questions are based on the concept of variation. The no of revolutions made depends on the circumference of the circle. Circumference depends on the radius of the circle. If radius is greater, lesser number of revolutions will be completed in the same time if the gears are interconnected. Since ratio of radius of X:Y is 4:10 = 2:5, No of revolutions made by X and Y will be in the ratio 5:2 (revolutions made are inversely proportional to the radius) 3000 revolutions/min = 50 revolutions/sec So gear Y will make 20 revolutions/sec Difference = 30
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Re: Gears X and Y are interconnected within and power
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28 Feb 2011, 10:09
Such problems can be quickly solved with a little visualization! Gear X: Circumference = 8 pi Revolutions per minute = 3000 Revolutions per second = 3000/60 = 50 For any point x on the circumference of gear x, distance travelled in one second = 50 * 8 pi Gear Y: Circumferenec = 20 pi For any point y on the circumference of gear y, the distance travelled in one second = 50 * 8 pi Therefore, revolutions per second = 50 * 8 pi / 20 pi = 20 Question: What is the difference between the number of revolutions per second... Ans: 50  20 = 30 Therefore, 'E'
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Re: Gears X and Y are interconnected within and power
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28 Feb 2011, 10:46
1 min: 4 inches> 3000 rev 1 inch> 3000* 4 rev 10 inch > 3000*4/10 = 1200 rev 4 inch: 1 min > 3000 rev 1 sec> 3000/60 = 50 rev 10 inch: 1 min > 1200 rev 1 sec > 1200/60 = 20 rev Diff: 5020 = 30 Ans: "E"
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Re: Gears X and Y are interconnected within and power
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28 Feb 2011, 20:18
Let y be the number of revs / sec by gear Y. The assumption is revs are inversely proportional to the diameter of the wheel. (1) 3000 revs/sec = 50 revs / sec From (1) 50 * 4 = y * 10 y = 20 Difference = 50  20 = 30 revs / sec



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Re: Gears X and Y are interconnected within and power
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20 Mar 2015, 17:49
VeritasPrepKarishma wrote: GMATD11 wrote: How to approach such type of question? Such questions are based on the concept of variation. The no of revolutions made depends on the circumference of the circle. Circumference depends on the radius of the circle. If radius is greater, lesser number of revolutions will be completed in the same time if the gears are interconnected. Since ratio of radius of X:Y is 4:10 = 2:5, No of revolutions made by X and Y will be in the ratio 5:2 (revolutions made are inversely proportional to the radius) 3000 revolutions/min = 50 revolutions/sec So gear Y will make 20 revolutions/sec Difference = 30 Doesn't this state that the diameters of X and Y are 4 and 10, respectively? So shouldn't we be using 2 and 5 for the radii?



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Gears X and Y are interconnected within and power
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Updated on: 21 Mar 2015, 06:24
First convert the frequency from revolutions per minute to revolutions per second.
\(\frac{3000}{60 seconds}=\frac{50}{second}\)
Since the diameter is directly proportional to the length of the circumference, and the circumference is inversely proportional to the frequency, the diameter is inversely proportional to the frequency
\(\frac{1}{4}:50=\frac{1}{10}:X \Rightarrow X=20\)
The difference of the two frequencies is \(5020=30\)
Answer E
Originally posted by ngie on 21 Mar 2015, 02:29.
Last edited by ngie on 21 Mar 2015, 06:24, edited 1 time in total.



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Re: Gears X and Y are interconnected within and power
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21 Mar 2015, 03:01
Simple question. I dont think this can be 700 questions especially when compared to several 700 levels questions that I saw in GMAT club. the solution 4 inches gear makes 3000 10 inches gear makes X apply chain rule. you will 4X3000/10 = 1200 (number of revs that 10 inch gear makes) Finally divide 3000 and 1200 by 60, you will get 50 and 20 respectively answer= 30. The only that I doubt in this question is, Is there a valid reason to think that a larger gear makes fewer revolutions. GMATD11 wrote: Gears X and Y are interconnected within and power
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Re: Gears X and Y are interconnected within and power
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21 Mar 2015, 11:07
Hi shriramvelamuri, This prompt is based on multishape Geometry, which is a category that consistently ranks at the 'higher end' of the difficulty spectrum on the GMAT. If you think that it's easy, then that's great, but most Test Takers would NOT find this question to be so simple. The prompt also involves rates; since the larger gear has a larger circumference, it has to turn through "more space" to complete a full rotation. By definition it will have fewer revolutions because its rate is based on the rate of the smaller gear (which has a smaller circumference and has to turn through "less space" to complete a full rotation). GMAT assassins aren't born, they're made, Rich
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Re: Gears X and Y are interconnected within and power
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23 Mar 2015, 01:20
metskj127 wrote: VeritasPrepKarishma wrote: GMATD11 wrote: How to approach such type of question? Such questions are based on the concept of variation. The no of revolutions made depends on the circumference of the circle. Circumference depends on the radius of the circle. If radius is greater, lesser number of revolutions will be completed in the same time if the gears are interconnected. Since ratio of radius of X:Y is 4:10 = 2:5, No of revolutions made by X and Y will be in the ratio 5:2 (revolutions made are inversely proportional to the radius) 3000 revolutions/min = 50 revolutions/sec So gear Y will make 20 revolutions/sec Difference = 30 Doesn't this state that the diameters of X and Y are 4 and 10, respectively? So shouldn't we be using 2 and 5 for the radii? Note that we are talking about the RATIO of the radii. The ratio of the radii will be the same as the ratio of the diameters. If the diameter of one circle is twice the diameter of another, then its radius will also be twice the radius of the other.
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Re: Gears X and Y are interconnected within and power
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18 Apr 2015, 22:35
shriramvelamuri wrote: Simple question. I dont think this can be 700 questions especially when compared to several 700 levels questions that I saw in GMAT club. the solution 4 inches gear makes 3000 10 inches gear makes X apply chain rule. you will 4X3000/10 = 1200 (number of revs that 10 inch gear makes) Finally divide 3000 and 1200 by 60, you will get 50 and 20 respectively answer= 30. The only that I doubt in this question is, Is there a valid reason to think that a larger gear makes fewer revolutions. GMATD11 wrote: Gears X and Y are interconnected within and power what is the chain rule here?I am messing up the calculation when i use the normal unitary method to find the number of revolutions made by 10 inch wheel and ending up with (3000*10)/4.



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Re: Gears X and Y are interconnected within and power
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20 Apr 2015, 02:42
Ralphcuisak wrote: shriramvelamuri wrote: Simple question. I dont think this can be 700 questions especially when compared to several 700 levels questions that I saw in GMAT club. the solution 4 inches gear makes 3000 10 inches gear makes X apply chain rule. you will 4X3000/10 = 1200 (number of revs that 10 inch gear makes) Finally divide 3000 and 1200 by 60, you will get 50 and 20 respectively answer= 30. The only that I doubt in this question is, Is there a valid reason to think that a larger gear makes fewer revolutions. GMATD11 wrote: Gears X and Y are interconnected within and power what is the chain rule here?I am messing up the calculation when i use the normal unitary method to find the number of revolutions made by 10 inch wheel and ending up with (3000*10)/4. You will multiply 3000 by 4/10 because a larger gear will make fewer revolutions. Since you want to reduce 3000, you will multiply it by 4/10 (a number smaller than 1). For more on this, check: http://www.veritasprep.com/blog/2013/02 ... variation/
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Gears X and Y are interconnected within and power
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02 Nov 2015, 06:38
tricky one! X makes 3000/60 > 50 revolutions Circumference is 2pi * r => circumference for gear X is 8pi and for Y is 20pi now we know that gear x makes 50 revolutions per second, or a total distance of 8pi*50, and this is the total distance gear y makes. to find how many revolutions gear y makes, divide 8pi*50/20pi > simplify pi's, 10,s and 2 with 4, and get 20. Gear Y makes 20 revolutions. Difference is thus 5020=30.
P.S. the proportion way of solving the problem is very interesting!!!



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Re: Gears X and Y are interconnected within and power
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03 Nov 2015, 16:26
gear X makes 50 rps ratio of gear X to gear Y circumferences=4⫪/10⫪= 2/5 (2/5)(50)=20 5020=30 rps difference between gear X and Y



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