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Relative speeds = 60:20
To have a gap of 120, we need to multiply the ratio by 3, as current gap = 40.

Thus 3 mins = 180 seconds.

Answer: Option D
ExpertsGlobal5
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. 2
B. 3
C. 120
D. 180
E. 360


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Speeds of gears
  • Gear X = 20 rotations/min
  • Gear Y = 60 rotations/min
Let's say after t minutes, Y has made exactly 120 more rotations than gear X

60t-20t=120
40t=120
t = 3 minutes = 180 seconds

Ans (D)

ExpertsGlobal5
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. 2
B. 3
C. 120
D. 180
E. 360


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In 1 min

- Gear X does 20 rotations.
- Gear Y does 60 rotations.

This means that ->

In 1 min, Gear Y makes 40 more rotations.

Then ->

To make 120 (40 x 3) more rotations, Gear Y should take 3 minutes i.e., 180 seconds. Choice D.

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ExpertsGlobal5
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. 2
B. 3
C. 120
D. 180
E. 360
D is the correct answer choice.

Video explanation:

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