The rate of spin of a certain gyroscope doubled every 15 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? Options:
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 6 times.   2^6 * x = 400   x = (2^4 * 25) / 2^6 = 25/4   The answer is 25/4.
<result>B</result>
The rate of spin of a certain gyroscope doubled every 15 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?