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Rates - GMATPrep Test 1

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Rates - GMATPrep Test 1

by kwah » Wed Feb 22, 2012 5:27 pm
I have attached a question I got incorrect on the GMATPrep Test. After review the steps I came up with are as follows...

P = 10 rev/min = 1/6 rev/sec
Q = 40 rev/min = 4/6 rev/sec

Therefore, we must solve for 'x' in the equation...

4/6x - 1/6x = 6

Answer: D

Is this the most efficient way to tackle this question? I would appreciate feedback, thank you.
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by Anurag@Gurome » Wed Feb 22, 2012 6:38 pm
kwah wrote:I have attached a question I got incorrect on the GMATPrep Test. After review the steps I came up with are as follows...

P = 10 rev/min = 1/6 rev/sec
Q = 40 rev/min = 4/6 rev/sec

Therefore, we must solve for 'x' in the equation...

4/6x - 1/6x = 6

Answer: D

Is this the most efficient way to tackle this question? I would appreciate feedback, thank you.
Yes, that's absolutely correct.

30 revolutions in 60 seconds.
So, 6 revolutions in (60/30) * 6 = 12 seconds
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by bubbliiiiiiii » Thu Feb 23, 2012 9:02 am
Hello Sir,

This seems more like remembering a formula.

Could you please describe the steps?
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by LalaB » Thu Feb 23, 2012 10:01 am
(1/40)*(x+6)=(1/10)*x
x=2

2+6=8 rev

so,

40 rev- 60 sec
8 rev-y sec

y=12

ans is 12
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by LalaB » Thu Feb 23, 2012 10:15 am
we can think this way too

Q 's work is 40 rev
P's work is 10 rev

so the rate of Rq/Rp =40/10=4/1

(x+6)/x=4/1
x=2

10 rev- 60 sec
2 rev -y sec

y=12
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by GMATGuruNY » Thu Feb 23, 2012 1:23 pm
bubbliiiiiiii wrote:Hello Sir,

This seems more like remembering a formula.

Could you please describe the steps?
Circular gears P and Q start rotating at the same time at constant speeds. Gear P makes 10 revolutions per minute, and gear Q makes 40 revolutions per minute. How many seconds after the gears start rotating will gear Q have made exactly 6 more revolutions than gear P?
When elements compete, SUBTRACT THEIR RATES.

Every minute Q makes 40 revolutions and P makes 10 revolutions.
The difference between their rates is 40-10 = 30.
Thus, every minute Q makes 30 more revolutions than P.

To determine how many seconds it will take Q to make 6 more revolutions than P, set up a proportion:
30 revolutions/60 seconds = 6 revolutions/x seconds
30x = 360
x = 12.
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by bubbliiiiiiii » Thu Feb 23, 2012 9:04 pm
When elements compete, SUBTRACT THEIR RATES.
Thanks Mitch. I have another query.

As in speed problems we subtract(when in same direction) or add (when in opp. directions) speeds based on direction, doesn't the direction of rotation (clockwise/anticlockwise) matter here?

or are we assuming that both the gears are rotating in either clockwise or anticlockwise direction?
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