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# The rate of spin of a certain gyroscope doubled every 10

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Current Student
Joined: 29 Jan 2005
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The rate of spin of a certain gyroscope doubled every 10 [#permalink]  14 May 2005, 05:32
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The rate of spin of a certain gyroscope doubled every 10 seconds from the moment a particular stopwatch started. If after a minute and a half the gyroscope reached a speed of 400 meters per second, what was the speed, in meters per second, when the stopwatch was started?

(A)25/3
(B)25/4
(C)25/8
(D)25/16
(E)25/32
Senior Manager
Joined: 15 Mar 2005
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Re: PS: gyroscope spin [#permalink]  14 May 2005, 05:44
A minute and half = 90 sec. Since doubles every 10 seconds, 10 sec intervals in 90 sec = 90/10 = 9. Thus total increase = 2^9 = 512 times.
Thus initial speed = 400/512 = 25/32
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let x be the initial speed

the speed after ninety seconds wud be x*2^9 = 512 x

512 x = 400 ,
x = 25/32
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