In 2/4 minutes the number of revolutions a CD makes = 58 ½  = 117/2 
 Then 
In 1 minute the number of revolutions the CD will make = [(117/2)/(2/4) ] * 1  = (117 * 4)/(2 * 2)  = 117 
So it can seen that the CD revolves at the rate of 117 revolutions per minute. I hope this is the answer you were looking for. The procedure for solving this problem is easy for you to understand i hope.