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 6400 meters per second, what was the speed, in meters per second, when the stopwatch was started?
Choose the correct choice from the following answers
A) 25/2 B) 25/4 C) 25/8 D) 25/16 E) 25/32
Let x be the original speed when the stopwatch was started.   In 90 seconds, the speed doubled 9 times.   2^9 * x = 6400   x = (2^8 * 25) / 2^9 = 25/2   The answer is 25/2.
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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 6400 meters per second, what was the speed, in meters per second, when the stopwatch was started?