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GMAT Prep?? (Circular Rotations)

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by dferm » Sun May 04, 2008 3:33 pm
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?

A. 6
B. 8
C. 10
D. 12
E. 15

Please explain...

Got this question right but need a faster way...

Thanks.
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by Neo2000 » Sun May 04, 2008 9:54 pm
P = 10/60 or 1R/6sec
Q = 40/60 or 2R/3sec = 4R/6sec

which means Q gains 3Revolutions every 6 seconds. For Q to gain 6, we need another 6seconds to pass. Which means 2sets of 6seconds have passed or 12seconds.
Ans : 12seconds
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by ksh » Sun May 04, 2008 10:10 pm
For P 10/60 RPS = 1 Rev in 6 Sec

For Q 40/60 RPS = 4 rev in 6 Sec

In another 6 sec P will complete 2 Rev while Q will complete 8 Rev i.e. 6 more than P.

Hence, in 12 sec Q will be 6 Rev more than P.

Ans is D
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