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gmat prep q

by yvonne12 » Wed Mar 07, 2007 9:20 am
circular gears p and q start rotating at the same time at a constant speeds. gear p makes 10 revolutiojns per minute and gear q makes 40 revolutions per minute. How many seconds after the gears start rotating will gear q have mad exactly 6 more revolutions thangear p?

ans.12

please explain
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by ssiva » Wed Mar 07, 2007 10:16 am
p = 10 revs in 60 seconds
q = 40 revs in 60 seconds

in 1 second p does 10/60 = 1/6 revs
in 1 second q does 40/60 = 4/6 revs

in 1 second q does 4/6 - 1/6 = 3/6 = 1/2revs more than p
therefore how many seconds = 6 revs

6 * 1 = x * 1/2

x = 12
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