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GMAT Prep - Rates

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GMAT Prep - Rates

by tutonaranjo » Thu Sep 06, 2007 7:20 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

Answer is D. Appreciatte a detailed explanation! thanks guys!
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by ri2007 » Thu Sep 06, 2007 7:46 pm
Here is a simple way of doing this

P does 10 rotations in 1 mins while Q does 40
So Q does 30 rotations more per min than P

Now build a simple ratio (cross multilication equation)

60 sec - 30 rotations extra so
X sec - 6 rotations extra

X = (60*6)/ 30 =12
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thanks

by tutonaranjo » Fri Sep 07, 2007 6:10 am
thanks guys! very simple! got it!
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