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fitzpratik
Posts: 224
15 Apr 2019, 08:44
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D
Be sure to select an answer first to save it in the Error Log before revealing the correct answer (OA)!
Difficulty:
35%
(medium)
Question Stats:
71% (01:31) correct
29%
(01:40)
wrong
based on 296
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Circular gears X and Y start rotating at the same time at constant speeds. Gear X makes 20 rotations per minute and gear Y makes 60 rotations per minute. How many seconds after the gears start rotating will gear Y have made exactly 120 more rotations than gear X?
A. 3
B. 2
C. 120
D. 180
E. 360
Source: ExpertsGlobal
A. 3
B. 2
C. 120
D. 180
E. 360
Source: ExpertsGlobal
15 Apr 2019, 09:44
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fitzpratik
This question tests your ability to write on your pad and confirm you are answering what the test is asking. The rotations are given in minutes, but it asks how many seconds gear Y will have 120 more rotations. We can write multiples of 60 for y and 20 for x and see that in 3 minutes y will have 180 rotations, and x 60, with a difference of 120 rotations. 3 is not the correct answer however, as there are 180 seconds in 3 minutes. D.
16 Apr 2019, 12:09
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Work rate of X = 20/60 = 1/3 rotations per second
Work rate of Y = 60/60 = 1 rotation per second
We can think of this problem shrinking-distance problem, where two objects are moving in the same direction and we have to find the time/point where they will meet.
Here Y has to catchup with X, so work rate will be 1-\(\frac{1}{3}\) = \(\frac{2}{3}\) rotations per second
Time taken to do 120 rotations = \(\frac{120}{2}\)* 3 = 180 sec
Work rate of Y = 60/60 = 1 rotation per second
We can think of this problem shrinking-distance problem, where two objects are moving in the same direction and we have to find the time/point where they will meet.
Here Y has to catchup with X, so work rate will be 1-\(\frac{1}{3}\) = \(\frac{2}{3}\) rotations per second
Time taken to do 120 rotations = \(\frac{120}{2}\)* 3 = 180 sec
