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 3200 meters per second, what was the speed, in meters per second, when the stopwatch was started?
Select the correct option
A) 25 / 3 B) 25 / 4 C) 25 / 8 D) 25 / 16 E) 25 / 32

Rationale: "let x be the original speed when the stopwatch was started . in 90 seconds , the speed doubled 9 times . 2 ^ 9 * x = 3200 x = ( 2 ^ 7 * 25 ) / 2 ^ 9 = 25 / 4 the answer is b ."
Answer: B