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Rates - GMATPrep Test 1

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kwah: Senior | Next Rank: 100 Posts; Posts: 70; Joined: Wed Jan 04, 2012 5:15 pm

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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Source: Beat The GMAT — Problem Solving | Wed Feb 22, 2012 5:27 pm

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Anurag@Gurome: GMAT Instructor; Posts: 3835; Joined: Fri Apr 02, 2010 10:00 pmFollowed by: 523 members; Location: Milpitas, CA; Thanked: 1854 times; GMAT Score:770

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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bubbliiiiiiii: Legendary Member; Posts: 979; Joined: Tue Apr 14, 2009 1:38 amFollowed by: 12 members; Location: Hyderabad, India; Thanked: 49 times; GMAT Score:700

by bubbliiiiiiii » Thu Feb 23, 2012 9:02 am

Hello Sir,

This seems more like remembering a formula.

Could you please describe the steps?

Regards,

Pranay

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LalaB: Master | Next Rank: 500 Posts; Posts: 425; Joined: Wed Dec 08, 2010 9:00 amFollowed by: 7 members; Thanked: 56 times; GMAT Score:690

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

Happy are those who dream dreams and are ready to pay the price to make them come true.(c)

In order to succeed, your desire for success should be greater than your fear of failure.(c)

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LalaB: Master | Next Rank: 500 Posts; Posts: 425; Joined: Wed Dec 08, 2010 9:00 amFollowed by: 7 members; Thanked: 56 times; GMAT Score:690

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

Happy are those who dream dreams and are ready to pay the price to make them come true.(c)

In order to succeed, your desire for success should be greater than your fear of failure.(c)

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GMATGuruNY: GMAT Instructor; Posts: 15539; Joined: Tue May 25, 2010 12:04 pmFollowed by: 1906 members; Location: New York, NY; Thanked: 13060 times; GMAT Score:790

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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bubbliiiiiiii: Legendary Member; Posts: 979; Joined: Tue Apr 14, 2009 1:38 amFollowed by: 12 members; Location: Hyderabad, India; Thanked: 49 times; GMAT Score:700

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?

Regards,

Pranay

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